Shareholders' Equity (Narrative) (Details) (USD $)	1 Months Ended	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011
Equity, Class of Treasury Stock [Line Items]
Amount of common stock authorized for repurchase through March 2014	$ 2,100,000,000
Common stock acquired (shares)		6,500,000
Average cost per share of treasury shares repurchased		$ 54.95
Approximate cost of treasury shares repurchased		360,000,000
Unrealized pretax gains (losses) on securities included in AOCI				(49,000,000)	22,000,000
Deferred taxes on unrealized pretax gains (losses) on securities included in AOCI				20,000,000	9,000,000
Realized gains (losses) from sale of available-for-sale securities		$ 5,000,000	$ 19,000,000
March 2013 Repurchase Program [Member]
Equity, Class of Treasury Stock [Line Items]
Common stock acquired (shares)		0
March 2012 Share Repurchase Program [Member]
Equity, Class of Treasury Stock [Line Items]
Stock repurchase program, remaining number of shares authorized to be repurchased	0	0