Common Share Repurchases (Details) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) $ / shares shares
Treasury Stock Transactions, Excluding Value of Shares Reissued [Abstract]
Number of shares repurchased (in shares) | shares	2.5
Cost of shares repurchased	$ 350
Average cost per share repurchased (in dollars per share) | $ / shares	$ 141.84
Number of shares acquired to cover tax withholding costs and exercise costs (in shares) | shares	0.3
Cost of shares acquired to cover tax withholding costs and exercise costs	$ 51
Remaining capacity under share repurchase authorization	$ 4,210