Common Share Repurchases (Details) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($) $ / shares shares
Treasury Stock Transactions, Excluding Value of Shares Reissued [Abstract]
Number of shares repurchased (in shares) | shares	2.9
Cost of shares repurchased	$ 375
Average cost per share repurchased (in dollars per share) | $ / shares	$ 129.42
Number of shares acquired to cover tax withholding costs and exercise costs (in shares) | shares	0.4
Cost of shares acquired to cover tax withholding costs and exercise costs	$ 46
Remaining capacity under share repurchase authorization	$ 2,910