Earnings Per Share - Calculation of Basic and Diluted Earnings Per Share (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share, Basic [Abstract]
Net income	$ 16,865	$ 13,479	$ 12,773	$ 10,549	$ 17,443	$ 9,699	$ 7,644	$ 8,851	$ 53,666	$ 43,637	$ 29,353
Less allocated income to participating securities										142	22
Net income available to common shareholders - basic									53,666	43,495	29,331
Basic weighted-average common shares									18,646,128	18,373,033	18,166,261
Basic earnings per share	$ 0.90	$ 0.72	$ 0.69	$ 0.57	$ 0.94	$ 0.53	$ 0.42	$ 0.48	$ 2.88	$ 2.37	$ 1.62
Net income available to common shareholders - diluted	$ 16,865	$ 13,479	$ 12,773	$ 10,549	$ 17,364	$ 9,673	$ 7,618	$ 8,836	$ 53,666	$ 43,495	$ 29,331
Weighted average common shares									18,646,128	18,373,033	18,166,261
Stock options and performance shares									282,376	375,776	408,930
Diluted weighted average common shares									18,928,504	18,748,809	18,575,191
Diluted earnings per common share	$ 0.89	$ 0.71	$ 0.68	$ 0.56	$ 0.92	$ 0.52	$ 0.41	$ 0.47	$ 2.84	$ 2.32	$ 1.58