Stock Repurchase (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended	12 Months Ended
	Jan. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock Repurchase
Common stock authorized for repurchase, maximum (in shares)		100.0
Stock repurchase program year	3 years
Common stock remaining number of shares authorized to repurchase		25.5
Common stock repurchased (in shares)		1.0	0.5	0.0
Common stock repurchased, value		$ 54.4		$ 20.0
Weighted average price per share of common stock (in dollars per share)		$ 56.25		$ 41.36
Payment of income tax obligation on employee equity awards		$ 13.6	$ 5.1