Stockholders' Equity (Details) - USD ($) shares in Thousands, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Repurchases [Abstract]
Stock Repurchase Shares Authorized	3,300
Treasury Stock, Shares, Acquired	0	922	3,757
Payments of Dividends	$ 6.7	$ 6.7	$ 6.9