Common Stock (Details) - USD ($) shares in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	May. 31, 2015
Equity, Class of Treasury Stock [Line Items]
Payments for repurchase of common stock	$ 1,400,000,000	$ 800,000,000	$ 550,000,000
Common Stock
Equity, Class of Treasury Stock [Line Items]
Common stock repurchased	24.8	15.5
Stock repurchase program, authorized amount				$ 2,000,000,000
Stock repurchase program, remaining authorized repurchase amount	$ 1,200,000,000