Stock Repurchases (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 06, 2017	Aug. 19, 2015
Stock Repurchases [Abstract]
Stock Repurchase Program, Authorized Repurchase Amount				$ 150,000,000.0	$ 100,000,000
Stock Repurchased During Period, Shares	687,048	1,479,454	2,924,306
Payments for Repurchase of Common Stock	$ 45,732,000	$ 59,784,000	$ 81,653,000
Stock Repurchased During Period, Value	39,400,000	$ 65,300,000	$ 82,500,000
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 133,900,000