Stockholders' Equity (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Apr. 25, 2017	Jul. 21, 2015	Oct. 17, 2014	Oct. 21, 2008
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Repurchase Program, Number of Shares Authorized to be Repurchased (in shares)				3,000,000	3,000,000	3,000,000	6,000,000
Repurchases of common stock, shares	2,400,000	900,000	3,800,000
Repurchases of common stock	$ 113,161	$ 38,252	$ 117,680
Remaining Number of Shares Authorized to be Repurchased	2,000,000
Restricted stock unit withholdings, shares	135,000	105,000	85,000
Restricted stock unit withholdings	$ 6,415	$ 4,686	$ 2,629
Common Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Repurchases of common stock, shares	2,378,000	894,000	3,770,000
Restricted stock unit withholdings, shares	135,000	105,000	85,000