Quarterly Results (Unaudited) (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								6 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, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating revenues	$ 974	$ 906	$ 940	$ 1,341	$ 1,851	$ 1,608	$ 1,884	$ 2,043		$ 4,161	$ 7,386	$ 9,185
Operating income (loss)	316	229	163	424	742	457	493	(327)		1,132	1,365	(1,347)
Income (loss) from continuing operations	257	236	92	242	622	392	340	(459)		827	895	(1,880)
Net income (loss)	257	236	93	241	622	395	341	(461)		827	897	(1,900)
Net income (loss) attributable to controlling interest	$ 243	$ 218	$ 82	$ 235	$ 616	$ 386	$ 335	$ (472)		$ 778	$ 865	$ (1,839)
Per share earnings (loss) from continuing operations
Basic (in dollars per share)	$ 0.64	$ 0.59	$ 0.22	$ 0.64	$ 1.68	$ 1.06	$ 0.91	$ (1.30)		$ 2.08	$ 2.36	$ (5.08)
Diluted (in dollars per share)	$ 0.64	$ 0.59	$ 0.22	$ 0.64	$ 1.68	$ 1.06	$ 0.91	$ (1.30)		$ 2.08	$ 2.36	$ (5.08)
Weighted-average shares outstanding
Basic (in shares)	371	365	365	364	364	364	363	363		367	363	362
Diluted (in shares)	371	365	365	364	364	364	363	363		367	363	362
Aggregate loss on impairment of assets held for sale										$ 41	$ 700
Loss associated with the impairment	$ 52						$ 668	$ 507
Gain associated with insurance recoveries	$ 30						788
Gain on retirement of debt		$ 110	$ 38						$ 23	$ 148	23	$ (13)
Previously reported
Operating revenues		903	943	$ 1,341	$ 1,851	$ 1,608	1,884	2,043			7,386	9,174
Operating income (loss)		225	154	414	750	445	506	(321)			1,380	(1,378)
Income (loss) from continuing operations		244	87	256	617	327	347	(467)			824	(1,946)
Net income (loss)		244	88	255	617	330	348	(469)			826	(1,966)
Net income (loss) attributable to controlling interest		$ 229	$ 77	$ 249	$ 611	$ 321	$ 342	$ (483)			$ 791	$ (1,913)
Per share earnings (loss) from continuing operations
Basic (in dollars per share)		$ 0.62	$ 0.21	$ 0.68	$ 1.66	$ 0.87	$ 0.93	$ (1.32)			$ 2.16	$ (5.29)
Diluted (in dollars per share)		$ 0.62	$ 0.21	$ 0.68	$ 1.66	$ 0.87	$ 0.93	$ (1.32)			$ 2.16	$ (5.29)
Weighted-average shares outstanding
Gain on retirement of debt											$ 23	$ (13)
Adjusted
Operating revenues		$ 3	$ (3)									11
Operating income (loss)		4	9	$ 10	$ (8)	$ 12	$ (13)	$ (6)			(15)	31
Income (loss) from continuing operations		(8)	5	(14)	5	65	(7)	8			71	66
Net income (loss)		(8)	5	(14)	5	65	(7)	8			71	66
Net income (loss) attributable to controlling interest		$ (11)	$ 5	$ (14)	$ 5	$ 65	$ (7)	$ 11			$ 74	$ 74
Per share earnings (loss) from continuing operations
Basic (in dollars per share)		$ (0.03)	$ 0.01	$ (0.04)	$ 0.02	$ 0.19	$ (0.02)	$ 0.02			$ 0.20	$ 0.21
Diluted (in dollars per share)		$ (0.03)	$ 0.01	$ (0.04)	$ 0.02	$ 0.19	$ (0.02)	$ 0.02			$ 0.20	$ 0.21