15. Segment Reporting Our determination of reportable business segments considers the strategic operating units under which we make financial decisions, allocate resources and assess performance of our business. Our reportable business segments are retail electricity and retail natural gas. The retail electricity segment consists of electricity sales and transmission to residential and commercial customers. The retail natural gas segment consists of natural gas sales to, and natural gas transportation and distribution for, residential and commercial customers. Corporate and other consists of expenses and assets of the retail electricity and natural gas segments that are managed at a consolidated level such as general and administrative expenses. Asset optimization activities are also included in Corporate and other. For the three months ended March 31, 2019 and 2018 , we recorded asset optimization revenues of $23.4 million and $78.2 million and asset optimization cost of revenues of $20.8 million and $75.5 million , respectively, which are presented on a net basis in asset optimization revenues. The acquisitions of HIKO in 2018 had no impact on our reportable business segments as the portions of those acquisitions related to retail natural gas and retail electricity have been included in those existing business segments. We use retail gross margin to assess the performance of our operating segments. Retail gross margin is defined as operating income (loss) plus (i) depreciation and amortization expenses and (ii) general and administrative expenses, less (i) net asset optimization revenues (expenses), (ii) net (losses) gains on non-trading derivative instruments, and (iii) net current period cash settlements on non-trading derivative instruments. We deduct net (losses) gains on non-trading derivative instruments, excluding current period cash settlements, from the retail gross margin calculation in order to remove the non-cash impact of net gains and losses on these derivative instruments. Retail gross margin should not be considered an alternative to, or more meaningful than, operating income, as determined in accordance with GAAP. Below is a reconciliation of retail gross margin to income (loss) before income tax expense (in thousands): Three Months Ended March 31, 2019 2018 Reconciliation of Retail Gross Margin to Income (loss) before taxes Income (loss) before income tax expense $ 3,786 $ (48,298 ) Interest and other income (189 ) (201 ) Interest expense 2,223 2,245 Operating income (loss) 5,820 (46,254 ) Depreciation and amortization 12,155 13,019 General and administrative 29,476 30,047 Less: Net asset optimization revenues 2,552 2,687 Net, loss on non-trading derivative instruments (19,803 ) (36,712 ) Net, Cash settlements on non-trading derivative instruments 8,125 (14,882 ) Retail Gross Margin $ 56,577 $ 45,719 Financial data for business segments are as follows (in thousands): Three Months Ended March 31, 2019 Retail Electricity Retail Natural Gas Corporate and Other Eliminations Consolidated Total Revenues $ 182,092 $ 58,062 $ 2,552 $  $ 242,706 Retail cost of revenues 165,888 29,367   195,255 Less: Net asset optimization revenue  2,552  2,552 Net, (losses) gains on non-trading derivative instruments (21,942 ) 2,139   (19,803 ) Current period settlements on non-trading derivatives 8,173 (48 )   8,125 Retail Gross Margin $ 29,973 $ 26,604 $  $  $ 56,577 Total Assets at March 31, 2019 $ 2,046,946 $ 707,228 $ 353,782 $ (2,659,093 ) $ 448,863 Goodwill at March 31, 2019 $ 117,813 $ 2,530 $  $  $ 120,343 Three Months Ended March 31, 2018 Retail Electricity Retail Natural Gas Corporate and Other Eliminations Consolidated Total revenues $ 220,899 $ 63,102 $ 2,687 $  $ 286,688 Retail cost of revenues 249,547 40,329   289,876 Less: Net asset optimization revenue   2,687  2,687 Net, losses on non-trading derivative instruments (33,319 ) (3,393 )   (36,712 ) Current period settlements on non-trading derivatives (15,048 ) 166   (14,882 ) Retail Gross Margin $ 19,719 $ 26,000 $  $  $ 45,719 Total Assets at December 31, 2018 $ 1,857,790 $ 649,969 $ 361,697 $ (2,380,718 ) $ 488,738 Goodwill at December 31, 2018 $ 117,813 $ 2,530 $  $  $ 120,343