Capital Structure (Details) - Common Stock - USD ($) $ in Millions			1 Months Ended		12 Months Ended
	Oct. 24, 2014	Mar. 25, 2014	Dec. 31, 2016	Oct. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 21, 2014
Class of Stock [Line Items]
Repurchase program, authorized amount	$ 40.0	$ 20.0		$ 50.0	$ 0.0			$ 10.0
Share repurchase program, increase in shares authorized	$ 20.0	$ 10.0	$ 10.0	$ 10.0
Repurchases of common stock, shares					0	905,403	4,118,386
Repurchases of common stock						$ 3.8
Stock option exercises, shares					225,043	90,496
Stock option exercises					$ 1.5	$ 0.3