Net Income per Share (Details) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Earnings Per Share [Abstract]
Net income	$ 19,202	$ 14,065	$ 68,848	$ 56,471
Less distributed and undistributed earnings allocated to participating securities	337	330	1,233	1,283
Net Income (Loss) Available to Common Stockholders, Basic	$ 18,865	$ 13,735	$ 67,615	$ 55,188
Weighted average common shares outstanding basic (in shares)	20,473	20,843	20,555	20,895
Incremental Common Shares Attributable to Dilutive Effect of Share-based Payment Arrangements	148	13	119	11
Weighted Average Number of Shares Outstanding, Diluted	20,621	20,856	20,674	20,906