Equity (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Securities Repurchase Program
Repurchase of common stock	$ 18.5	$ 173.6	$ 48.9
Acquired Employee Equity Awards
Securities Repurchase Program
Repurchased shares	133,710	144,717	156,383
Repurchase of common stock	$ 5.8	$ 5.7	$ 8.0
Repurchased shares, average price (in dollars per share)	$ 43.28	$ 39.47	$ 51.05
Share Repurchase Program
Securities Repurchase Program
Repurchased shares	302,000	4,512,325	854,313
Repurchase of common stock	$ 12.7	$ 167.9	$ 40.9
Repurchased shares, average price (in dollars per share)	$ 41.95	$ 37.21	$ 47.86
Remaining amount authorized to be repurchased	$ 200.0
Common Stock
Securities Repurchase Program
Repurchased shares	435,710	4,657,042	1,010,696