Net Income Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Basic:
Net income	$ 27,772	$ 24,145	$ 54,789	$ 47,467
Average common shares outstanding (in shares)	34,129	33,588	34,099	33,554
Basic net income per common share (in dollars per share)	$ 0.81	$ 0.72	$ 1.61	$ 1.41
Diluted:
Adjustment for common share equivalents (in shares)	195	595	205	592
Average common shares outstanding-diluted (in shares)	34,324	34,183	34,304	34,146
Diluted net income per common share (in dollars per share)	$ 0.81	$ 0.71	$ 1.60	$ 1.39
Anti-dilutive shares (in shares)	7	7	7	7