Consolidated Statements Of Changes In Shareholders' Equity - USD ($) $ in Millions	Total	Preferred Stock [Member]	Common Stock [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 7,507	$ 828	$ 4,767	$ 1,967	$ (55)
Balance (in shares) at Dec. 31, 2015			204,417,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income for the period	91			91
Other comprehensive income, net of tax	43				43
Net activity under employee plans and related tax benefits	11		$ 11
Net activity under employee plans and related tax benefits, shares			127,000
Dividends on preferred stock	(12)			(12)
Dividends on common stock	(13)			(13)
Change in deferred compensation	(2)			(2)
Balance (in shares) at Mar. 31, 2016			204,544,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	7,625	828	$ 4,778	2,031	(12)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	7,634	710	4,725	2,321	(122)
Stock Repurchased and Retired During Period, Value	$ (45)		$ (45)
Stock Repurchased During Period, Shares	1,100,000		(1,060,000)
Balance (in shares) at Dec. 31, 2016	203,085,000		203,085,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income for the period	$ 139			139
Other comprehensive income, net of tax	11				11
Net activity under employee plans and related tax benefits	16		$ 16
Net activity under employee plans and related tax benefits, shares			570,000
Dividends on preferred stock	(10)			(10)
Dividends on common stock	$ (15)			(15)
Increase (Decrease) in Deferred Compensation				0
Balance (in shares) at Mar. 31, 2017	202,595,000		202,595,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 7,730	$ 710	$ 4,696	$ 2,435	$ (111)