Business Segments Our operations are organized into two reportable segments: (i) Logistics and Marketing and (ii) Gathering and Processing. These segments are monitored separately by management for performance against our internal forecast and are consistent with internal financial reporting. These segments have been identified based on the differing products and services, regulatory environment and the expertise required for these operations. Our Gathering and Processing reportable segment includes operating segments that have been aggregated based on the nature of the products and services provided. Gross margin is a performance measure utilized by management to monitor the operations of each segment. The accounting policies of the reportable segments are the same as those described in the summary of significant accounting policies included in Note 2 - Summary of Significant Accounting Policies. Our Logistics and Marketing segment includes transporting, trading, marketing, and storing natural gas and NGLs, fractionating NGLs, and wholesale propane logistics. Our Gathering and Processing segment consists of gathering, compressing, treating, processing natural gas, producing and fractionating NGLs, and recovering condensate. The remainder of our business operations is presented as Other, and consists of unallocated corporate costs. Elimination of inter-segment transactions are reflected in the eliminations column. The following tables set forth our segment information: Year Ended December 31, 2018 : Logistics and Marketing Gathering and Processing Other Eliminations Total (millions) Total operating revenue $ 9,014 $ 5,843 $  $ (5,035 ) $ 9,822 Gross margin (a) $ 225 $ 1,578 $  $  $ 1,803 Operating and maintenance expense (47 ) (692 ) (21 )  (760 ) Depreciation and amortization expense (15 ) (346 ) (27 )  (388 ) General and administrative expense (12 ) (19 ) (245 )  (276 ) Asset impairments  (145 )   (145 ) Other expense, net (4 ) (6 ) (1 )  (11 ) Loss from financing activities   (19 )  (19 ) Earnings from unconsolidated affiliates 362 8   370 Interest expense   (269 )  (269 ) Income tax expense   (3 )  (3 ) Net income (loss) $ 509 $ 378 $ (585 ) $  $ 302 Net income attributable to noncontrolling interests  (4 )   (4 ) Net income (loss) attributable to partners $ 509 $ 374 $ (585 ) $  $ 298 Non-cash derivative mark-to-market (b) $ (4 ) $ 112 $  $  $ 108 Capital expenditures $ 8 $ 570 $ 17 $  $ 595 Investments in unconsolidated affiliates, net $ 350 $ 4 $  $  $ 354 Year Ended December 31, 2017 : Logistics and Marketing Gathering and Processing Other Eliminations Total (millions) Total operating revenue $ 7,757 $ 5,467 $  $ (4,762 ) $ 8,462 Gross margin (a) $ 200 $ 1,377 $  $  $ 1,577 Operating and maintenance expense (41 ) (602 ) (18 )  (661 ) Depreciation and amortization expense (14 ) (343 ) (22 )  (379 ) General and administrative expense (11 ) (19 ) (260 )  (290 ) Asset impairments  (48 )   (48 ) Other expense (11 )    (11 ) Gain on sale of assets, net  34   34 Earnings from unconsolidated affiliates 243 60   303 Interest expense   (289 )  (289 ) Income tax expense   (2 )  (2 ) Net income (loss) $ 366 $ 459 $ (591 ) $  $ 234 Net income attributable to noncontrolling interests  (5 )   (5 ) Net income (loss) attributable to partners $ 366 $ 454 $ (591 ) $  $ 229 Non-cash derivative mark-to-market (b) $ (4 ) $ (24 ) $  $  $ (28 ) Non-cash lower of cost or market adjustments $ 2 $  $  $  $ 2 Capital expenditures $ 3 $ 350 $ 22 $  $ 375 Investments in unconsolidated affiliates, net $ 147 $ 1 $  $  $ 148 Year Ended December 31, 2016 : Logistics and Marketing Gathering and Processing Other Eliminations Total (millions) Total operating revenue $ 6,186 $ 4,490 $  $ (3,783 ) $ 6,893 Gross margin (a) $ 205 $ 1,227 $  $  $ 1,432 Operating and maintenance expense (43 ) (611 ) (16 )  (670 ) Depreciation and amortization expense (15 ) (344 ) (19 )  (378 ) General and administrative expense (9 ) (14 ) (269 )  (292 ) Other (expense) income (5 ) 73 (3 )  65 Gain on sale of assets, net 16 19   35 Restructuring costs   (13 )  (13 ) Earnings from unconsolidated affiliates 209 73   282 Interest expense   (321 )  (321 ) Income tax expense   (46 )  (46 ) Net income (loss) $ 358 $ 423 $ (687 ) $  $ 94 Net income attributable to noncontrolling interests  (6 )   (6 ) Net income (loss) attributable to partners $ 358 $ 417 $ (687 ) $  $ 88 Non-cash derivative mark-to-market (b) $ (20 ) $ (119 ) $  $  $ (139 ) Non-cash lower of cost or market adjustments $ 3 $  $  $  $ 3 Capital expenditures $ 10 $ 107 $ 27 $  $ 144 Investments in unconsolidated affiliates, net $ 52 $ 1 $  $  $ 53 December 31, December 31, 2018 2017 (millions) Segment long-term assets: Gathering and Processing $ 9,058 $ 8,943 Logistics and Marketing 3,661 3,348 Other (c) 276 265 Total long-term assets 12,995 12,556 Current assets 1,271 1,322 Total assets $ 14,266 $ 13,878 (a) Gross margin consists of total operating revenues, including commodity derivative activity, less purchases and related costs. Gross margin is viewed as a non-GAAP financial measure under the rules of the SEC, but is included as a supplemental disclosure because it is a primary performance measure used by management as it represents the results of product sales versus product purchases. As an indicator of our operating performance, gross margin should not be considered an alternative to, or more meaningful than, net income or net cash provided by operating activities as determined in accordance with GAAP. Our gross margin may not be comparable to a similarly titled measure of another company because other entities may not calculate gross margin in the same manner. (b) Non-cash commodity derivative mark-to-market is included in gross margin, along with cash settlements for our commodity derivative contracts. (c) Other long-term assets not allocable to segments consist of corporate leasehold improvements and other long-term assets.