In Millions Years Ended December 31 2015 2014 2013 CMS Energy, including Consumers Operating revenue Electric utility $ 4,249 $ 4,436 $ 4,173 Gas utility 1,916 2,363 2,148 Enterprises 190 299 181 Other reconciling items 101 81 64 Total operating revenue  CMS Energy $ 6,456 $ 7,179 $ 6,566 Consumers Operating revenue Electric utility $ 4,249 $ 4,436 $ 4,173 Gas utility 1,916 2,363 2,148 Other reconciling items - 1 - Total operating revenue  Consumers $ 6,165 $ 6,800 $ 6,321 CMS Energy, including Consumers Depreciation and amortization Electric utility $ 567 $ 522 $ 484 Gas utility 177 156 138 Enterprises 4 4 3 Other reconciling items 2 3 3 Total depreciation and amortization  CMS Energy $ 750 $ 685 $ 628 Consumers Depreciation and amortization Electric utility $ 567 $ 522 $ 484 Gas utility 177 156 138 Total depreciation and amortization  Consumers $ 744 $ 678 $ 622 CMS Energy, including Consumers Income from equity method investees 1 Enterprises $ 14 $ 15 $ 13 Total income from equity method investees  CMS Energy $ 14 $ 15 $ 13 CMS Energy, including Consumers Interest charges Electric utility $ 178 $ 181 $ 179 Gas utility 71 67 64 Other reconciling items 147 159 155 Total interest charges  CMS Energy $ 396 $ 407 $ 398 Consumers Interest charges Electric utility $ 178 $ 181 $ 179 Gas utility 71 67 64 Other reconciling items 1 2 2 Total interest charges  Consumers $ 250 $ 250 $ 245 In Millions Years Ended December 31 2015 2014 2013 CMS Energy, including Consumers Income tax expense (benefit) Electric utility $ 224 $ 211 $ 242 Gas utility 78 95 104 Enterprises 3 (1) (4) Other reconciling items (34) (55) (40) Total income tax expense  CMS Energy $ 271 $ 250 $ 302 Consumers Income tax expense Electric utility $ 224 $ 211 $ 242 Gas utility 78 95 104 Total income tax expense  Consumers $ 302 $ 306 $ 346 CMS Energy, including Consumers Net income (loss) available to common stockholders Electric utility $ 437 $ 384 $ 363 Gas utility 154 179 168 Enterprises 4 (1) 2 Other reconciling items (72) (85) (81) Total net i