Segment Data We aggregate our operating segments into two reportable segments: (i) pipelines and transportation and (ii) wholesale marketing and terminalling:  The assets and investments reported in the pipelines and transportation segment provide crude oil gathering and crude oil, intermediate and refined products transportation and storage services to Delek Holdings' refining operations and independent third parties.  The wholesale marketing and terminalling segment provides wholesale marketing and terminalling services to Delek Holdings' refining operations and independent third parties. Our operating segments adhere to the accounting policies used for our consolidated financial statements. Our operating segments are managed separately because each segment requires different industry knowledge, technology and marketing strategies. Decisions concerning the allocation of resources and assessment of operating performance are made based on this segmentation. Management measures the operating performance of each of its reportable segments based on segment contribution margin. Segment contribution margin is defined as net revenues less cost of materials and other and operating expenses, excluding depreciation and amortization. The following is a summary of business segment operating performance as measured by contribution margin for the periods indicated (in thousands): Three Months Ended September 30, Nine Months Ended September 30, 2019 2018 2019 2018 Pipelines and Transportation Net revenues: Affiliate $ 39,304 $ 36,132 $ 112,694 $ 99,624 Third party 5,281 3,653 16,733 11,618 Total pipelines and transportation 44,585 39,785 129,427 111,242 Cost of materials and other 4,947 5,055 17,871 14,691 Operating expenses (excluding depreciation and amortization) 12,547 9,499 36,109 29,054 Segment contribution margin $ 27,091 $ 25,231 $ 75,447 $ 67,497 Capital spending (1) $ 3,103 $ 2,086 $ 4,345 $ 4,320 Wholesale Marketing and Terminalling Net revenues: Affiliate (2) $ 27,343 $ 27,703 $ 78,836 $ 78,935 Third party 65,628 96,622 237,119 308,134 Total wholesale marketing and terminalling 92,971 124,325 315,955 387,069 Cost of materials and other 67,647 100,541 244,842 315,953 Operating expenses (excluding depreciation and amortization) 5,888 5,896 15,711 13,835 Segment contribution margin $ 19,436 $ 17,888 $ 55,402 $ 57,281 Capital spending (1) $ 889 $ 905 $ 1,893 $ 3,120 Consolidated Net revenues: Affiliate $ 66,647 $ 63,835 $ 191,530 $ 178,559 Third party 70,909 100,275 253,852 319,752 Total Consolidated 137,556 164,110 445,382 498,311 Cost of materials and other 72,594 105,596 262,713 330,644 Operating expenses (excluding depreciation and amortization presented below) 18,435 15,395 51,820 42,889 Contribution margin $ 46,527 $ 43,119 $ 130,849 $ 124,778 General and administrative expenses 5,280 3,076 15,046 9,798 Depreciation and amortization 6,588 6,702 19,801 19,721 (Gain) loss on asset disposals (70 ) 717 (95 ) 648 Operating income $ 34,729 $ 32,624 $ 96,097 $ 94,611 Capital spending (1) $ 3,992 $ 2,991 $ 6,238 $ 7,440 (1) Capital spending excludes transaction costs capitalized in the amount of $0.4 million that relate to the Big Spring Logistic Assets Acquisition for the nine months ended September 30, 2018 . (2) Affiliate revenue for the wholesale marketing and terminalling segment is presented net of amortization expense pertaining to the Marketing Contract Intangible Acquisition. See Note 3 for additional information. The following table summarizes the total assets for each segment as of September 30, 2019 and December 31, 2018 (in thousands): September 30, 2019 December 31, 2018 Pipelines and transportation $ 529,219 $ 387,333 Wholesale marketing and terminalling 238,588 237,260 Total assets $ 767,807 $ 624,593 Property, plant and equipment and accumulated depreciation as of September 30, 2019 and depreciation expense by reporting segment for the three and nine months ended September 30, 2019 were as follows (in thousands): Pipelines and Transportation Wholesale Marketing and Terminalling Consolidated Property, plant and equipment $ 361,478 $ 96,238 $ 457,716 Less: accumulated depreciation (122,397 ) (37,226 ) (159,623 ) Property, plant and equipment, net $ 239,081 $ 59,012 $ 298,093 Depreciation expense for the three months ended September 30, 2019 $ 5,240 $ 1,348 $ 6,588 Depreciation expense for the nine months ended September 30, 2019 $ 15,784 $ 4,017 $ 19,801 In accordance with ASC 360, Property, Plant &amp; Equipment , we evaluate the realizability of property, plant and equipment as events occur that might indicate potential impairment. There were no indicators of impairment of our property, plant and equipment as of September 30, 2019 .