Capital Stock (Schedule of Shares Repurchased) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Equity [Abstract]
Repurchase of common stock (in shares)	10,682,419	22,452,599	16,652,913
Aggregate cost of shares repurchased	$ 554	$ 939	$ 600
Average price of repurchased shares (USD per share)	$ 51.83	$ 41.79	$ 36.05