Stock Repurchases, Dividends and Earnings per Share - Share Repurchase Activity (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Equity [Abstract]
Number of shares repurchased (in shares)	1.5	3.8
Amount paid	$ 111.5	$ 254.5
Weighted average cost per share (in dollars per share)	$ 74.76	$ 66.69