Stockholders' Equity (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jun. 13, 2016	Jan. 16, 2015
Equity, Class of Treasury Stock [Line Items]
Stock Repurchase Program, Authorized Amount				$ 150,000	$ 100,000
Stock Repurchased and Retired During Period, Shares			43,000
Payments for Repurchase of Equity			$ (1,600)
sharesreceivedforexercisepriceofoptions			46,174
payment exercise of stock option			$ 2,200
Stock Repurchased and Retired During Period, Value	$ (58,070)	$ (43,091)	$ (3,819)
Stock Issued During Period, Shares, Acquisitions			794,700
Stock Issued During Period, Value, Acquisitions			$ 30,183
Stock Repurchase Program, Remaining Authorized Repurchase Amount	$ 48,800
Common Stock [Member]
Equity, Class of Treasury Stock [Line Items]
Stock Repurchased and Retired During Period, Shares	1,229,249	1,133,553	89,174
Stock Repurchased and Retired During Period, Value	$ (12)	$ (11)	$ (1)
Stock Issued During Period, Shares, Acquisitions			794,700
Stock Issued During Period, Value, Acquisitions			$ 8
Additional Paid-in Capital [Member]
Equity, Class of Treasury Stock [Line Items]
Payments for Repurchase of Equity			(400)
payment exercise of stock option			800
Stock Repurchased and Retired During Period, Value	(13,229)	(11,885)	(1,173)
Stock Issued During Period, Value, Acquisitions			30,175
Retained Earnings [Member]
Equity, Class of Treasury Stock [Line Items]
Payments for Repurchase of Equity			(1,200)
payment exercise of stock option			1,400
Stock Repurchased and Retired During Period, Value	$ (44,829)	$ (31,195)	$ (2,645)