15. Segment Reporting The Companys determination of reportable business segments considers the strategic operating units under which the Company makes financial decisions, allocates resources and assesses performance of its retail and asset optimization businesses. The Companys reportable business segments are retail natural gas and retail electricity. The retail natural gas segment consists of natural gas sales to, and natural gas transportation and distribution for, residential and commercial customers. Asset optimization activities, considered an integral part of securing the lowest price natural gas to serve retail gas load, are part of the retail natural gas segment. The Company recorded asset optimization revenues of $78.2 million and $61.9 million and asset optimization cost of revenues of $75.5 million and $62.1 million for the three months ended March 31, 2018 and 2017 , respectively, which are presented on a net basis in asset optimization revenues. The retail electricity segment consists of electricity sales and transmission to residential and commercial customers. Corporate and other consists of expenses and assets of the retail natural gas and retail electricity segments that are managed at a consolidated level such as general and administrative expenses. The acquisitions of Perigee and the Verde Companies in 2017 and the acquisition 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. To assess the performance of the Companys operating segments, the Chief Operating Decision Maker analyzes retail gross margin. The Company defines retail gross margin 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 gains (losses) on non-trading derivative instruments, and (iii) net current period cash settlements on non-tradin