Earnings Per Share - Calculation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Earnings Per Share, Basic [Abstract]
Net income available to common shareholders - basic	$ 14,319	$ 12,773	$ 29,449	$ 23,322
Basic weighted average common shares	18,917,229	18,600,186	18,882,693	18,566,235
Basic earnings per common share	$ 0.76	$ 0.69	$ 1.56	$ 1.26
Net income available to common shareholders - diluted	$ 14,319	$ 12,773	$ 29,449	$ 23,322
Weighted average common shares	18,917,229	18,600,186	18,882,693	18,566,235
Stock options and performance shares	162,836	294,699	187,256	319,149
Diluted weighted average common shares	19,080,065	18,894,885	19,069,949	18,885,384
Diluted earnings per common share	$ 0.75	$ 0.68	$ 1.54	$ 1.23