CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
CONDENSED CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME
Net income (loss)	$ 257	$ 236	$ 93	$ 241	$ 622	$ 395	$ 341	$ (461)	$ 827	$ 897	$ (1,900)
Net income (loss) attributable to noncontrolling interest									49	32	(61)
Net income (loss) attributable to controlling interest	$ 243	$ 218	$ 82	$ 235	$ 616	$ 386	$ 335	$ (472)	778	865	(1,839)
Components of net periodic benefit costs before reclassifications									(20)	63	(170)
Components of net periodic benefit costs reclassified to net income									8	23	17
Gain on derivative instruments reclassified to net income											(2)
Other comprehensive income (loss) before income taxes									(12)	86	(155)
Income taxes related to other comprehensive income									6	(17)	35
Other comprehensive income (loss)									(6)	69	(120)
Other comprehensive income attributable to controlling interest									(6)	69	(120)
Total comprehensive income (loss)									821	966	(2,020)
Total comprehensive income (loss) attributable to noncontrolling interest									49	32	(61)
Total comprehensive income attributable to controlling interest									$ 772	$ 934	$ (1,959)