Stockholders' Equity (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Equity, Class of Treasury Stock [Line Items]
Treasury stock purchase plan	$ 40.0
Treasury stock purchase plan remaining available	$ 7.2
Stock Repurchased and Retired During Period, Shares		$ 15.4
Shares Paid for Tax Withholding for Share Based Compensation	27,636	20,261
Payments Related to Tax Withholding for Share-based Compensation	$ 4.1	$ 1.9
Treasury Stock, Common, Shares	0	93