Condensed Financial Information Condensed Statement of Cash Flows (Parent Only) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Net income	$ 129	$ 147	$ 193	$ 145	$ 127	$ 139	$ 133	$ 153	$ 614	$ 552	$ 268
Depreciation and amortization									262	239	325
Equity in undistributed (income) loss from subsidiaries									(2)	5	(9)
Losses on early extinguishment of debt									9	0	37
Other									(7)	(5)	1
Net Cash Provided by (Used in) Operating Activities, Continuing Operations									1,121	1,646	843
Capital expenditures for property and equipment									(102)	(75)	(70)
Other									43	1	(73)
Net Cash Provided by (Used in) Investing Activities, Continuing Operations									(13,576)	(4,004)	2
Proceeds from issuance of senior notes									999	0	460
Payments on senior notes									(1,000)	0	(800)
Issuance of preferred stock									300	400	0
Repurchases of common stock									(362)	(452)	(50)
Preferred stock dividends									25	0	0
Other									(36)	(28)	(19)
Net Cash Provided by (Used in) Financing Activities, Continuing Operations									11,436	2,075	(395)
Cash and Cash Equivalents, Period Increase (Decrease)									(1,019)	(283)	450
Cash and equivalents	931				1,950				931	1,950	2,233	$ 1,783
Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net income									614	552	268
Depreciation and amortization									51	48	44
Equity in undistributed (income) loss from subsidiaries									573	323	(635)
Losses on early extinguishment of debt									9	0	5
Other									213	585	(152)
Net Cash Provided by (Used in) Operating Activities, Continuing Operations									314	862	800
Capital expenditures for property and equipment									(59)	(36)	(33)
Cash contributions to subsidiaries									(61)	(766)	(147)
Other									6	16	0
Net Cash Provided by (Used in) Investing Activities, Continuing Operations									(114)	(786)	(180)
Proceeds from issuance of senior notes									999	0	460
Payments on senior notes									(1,000)	0	(800)
Issuance of preferred stock									300	400	0
Repurchases of common stock									(362)	(452)	(50)
Preferred stock dividends									25
Other									(35)	(40)	(18)
Net Cash Provided by (Used in) Financing Activities, Continuing Operations									(123)	(92)	(408)
Cash and Cash Equivalents, Period Increase (Decrease)									77	(16)	212
Cash and equivalents	$ 493				$ 416				$ 493	$ 416	$ 432	$ 220