Common Stock (Details) (USD $) Share data in Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	May 31, 2014
Equity, Class of Treasury Stock [Line Items]
Payments for repurchase of common stock	$ 800,000,000	$ 550,000,000	$ 230,000,000
Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Common stock repurchased	15.5	13.2
Stock repurchase program, authorized amount				2,000,000,000
Stock repurchase program, remaining authorized repurchase amount	$ 1,300,000,000