Net Income Per Common Share - (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Basic
Net income applicable to common stock	$ 15,394	$ 14,853	$ 30,634	$ 28,450
Average common shares outstanding	31,626,059	31,496,737	31,601,304	31,466,610
Net income per common share - basic (usd per share)	$ 0.49	$ 0.47	$ 0.97	$ 0.90
Diluted
Net income applicable to common stock	$ 15,394	$ 14,853	$ 30,634	$ 28,450
Average common shares outstanding	31,626,059	31,496,737	31,601,304	31,466,610
Effect of dilutive stock-based compensation (shares)	239,113	201,461	232,953	215,886
Average common shares outstanding - diluted (shares)	31,865,172	31,698,198	31,834,257	31,682,496
Net income per common share - diluted (usd per share)	$ 0.48	$ 0.47	$ 0.96	$ 0.90