Stock Repurchase (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Stock Repurchase
Common stock authorized for repurchase, maximum (in shares)	250.0
Common stock remaining number of shares authorized to repurchase	159.6
Common stock repurchased (in shares)	1.7	1.0	0.5
Common stock repurchased, value	$ 94.6	$ 54.4	$ 20.0
Weighted average price per share of common stock (in dollars per share)	$ 56.86	$ 56.25	$ 41.36
Payment of income tax obligation on employee equity awards	$ 9.3	$ 13.6	$ 5.1