Condensed Statement of Cash Flows (Parent Company Only) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Cash flows from operating activities:
Net income	$ 89	$ (153)	$ 292	$ 40	$ 41	$ 86	$ 69	$ 97	$ 268	$ 293	$ 86
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Depreciation and amortization									325	331	395
Equity in undistributed (income) loss from subsidiaries									(7)	(3)	(4)
Losses on early extinguishment of debt									37	6	0
Other									(6)	(2)	(1)
Net cash provided by operating activities									832	701	1,117
Cash flows from investing activities:
Capital expenditures for property and equipment									(70)	(87)	(47)
Other									73	(69)	6
Net cash used in investing activities									2	1,713	286
Cash flows from financing activities:
Net proceeds from issuance of senior notes									460	540	0
Payments on senior notes									(800)	(940)	0
Repurchases of common stock									(50)	0	0
Other									(8)	53	2
Net cash provided by (used in) financing activities									(384)	(2,469)	(2,327)
Increase (decrease) in cash and equivalents									450	(55)	(924)
Cash and equivalents, beginning of period				1,783				1,838	1,783	1,838	2,762
Cash and equivalents, end of period	2,233				1,783				2,233	1,783	1,838
Parent Company [Member]
Cash flows from operating activities:
Net income									268	293	86
Adjustments to reconcile net income (loss) to net cash provided by (used in) operating activities:
Depreciation and amortization									44	38	40
Equity in undistributed (income) loss from subsidiaries									635	(148)	(3)
Losses on early extinguishment of debt									5	6	0
Other									(163)	(28)	(60)
Net cash provided by operating activities									789	161	63
Cash flows from investing activities:
Capital expenditures for property and equipment									(33)	(62)	(24)
Proceeds from sale of subsidiary									0	76	0
Cash contributions to subsidiaries									(147)	(29)	(39)
Other									0	0	4
Net cash used in investing activities									(180)	(15)	(59)
Cash flows from financing activities:
Net proceeds from issuance of senior notes									460	540	0
Payments on senior notes									(800)	(940)	0
Repurchases of common stock									(50)	0	0
Other									(7)	68	2
Net cash provided by (used in) financing activities									(397)	(332)	2
Increase (decrease) in cash and equivalents									212	(186)	6
Cash and equivalents, beginning of period				$ 220				$ 406	220	406	400
Cash and equivalents, end of period	$ 432				$ 220				$ 432	$ 220	$ 406