Common Stock - Repurchases (Details) - Common stock - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Class of Stock [Line Items]
Total number of shares of common stock repurchased (in shares)	89.8	82.3	96.1
Aggregate purchase price of common stock repurchases	$ 5,616	$ 4,760	$ 4,789