Share Repurchase Program (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Number of shares repurchased	13.7	13.3	13.6
Aggregate cost, including fees	$ 300.1	$ 303.3	$ 285.2
Average price per share, including fees	$ 21.97	$ 22.76	$ 20.97
2017 Share Repurchase Program
Share repurchase program, initial authorized amount	$ 300.0
Stock repurchase program, remaining authorized repurchase amount	$ 155.5