Reconciliation of Basic and Diluted Earnings per Share Computations (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 Disclosure [Line Items]
Net income	$ 19,275	$ 31,225	$ 16,861	$ 27,968	$ 14,333	$ 21,286	$ 13,937	$ 13,910	$ 95,329	$ 63,466	$ 63,762
Basic weighted average shares									14,259,768	14,133,925	14,052,311
Effect of common stock equivalents:
Assumed exercise of stock options and vesting of stock grants									386,601	211,628	275,509
Diluted weighted average shares									14,646,369	14,345,553	14,327,820
Net income per common share:
Basic	$ 1.35	$ 2.19	$ 1.18	$ 1.96	$ 1.01	$ 1.50	$ 0.99	$ 0.99	$ 6.69	$ 4.49	$ 4.54
Diluted	$ 1.31	$ 2.14	$ 1.17	$ 1.94	$ 1.00	$ 1.48	$ 0.97	$ 0.97	$ 6.51	$ 4.42	$ 4.45