Capital Stock (Schedule of Shares Repurchased) (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity [Abstract]
Total number of shares repurchased (in shares)	41,604,141	16,221,001	10,682,419
Aggregate cost of shares repurchased	$ 2,917	$ 1,030	$ 554
Average price per share of shares repurchased (USD per share)	$ 70.10	$ 63.48	$ 51.83