Summary of Stock Incentive Plan Activity (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Non-vested stock options outstanding - Number of Shares
Beginning Balance	284,925	281,975	284,075
Stock options granted	40,000	146,000	130,000
Stock options vested	(125,071)	(142,050)	(95,700)
Stock options forfeited	(41,000)	(1,000)	(36,400)
Ending Balance	158,854	284,925	281,975
Non-vested stock options outstanding - Weighted-Average grant fair value
Beginning Balance	$ 15.56	$ 12.59	$ 12.94
Stock options granted	$ 20.60	$ 21.27	$ 12.49
Stock options vested	$ 15.31	$ 14.78	$ 13.85
Stock options forfeited	$ 16.68	$ 16.08	$ 12.55
Ending Balance	$ 18.81	$ 15.56	$ 12.59
Stock Options
Stock options outstanding - Number of shares
Beginning Balance			561,300
Stock options granted	40,000	146,000	130,000
Stock options exercised			(40,550)
Stock options exercised	$ (72,050)	$ (27,500)
Stock options forfeited	(41,000)	(4,000)	(45,150)
Ending Balance	647,050	720,100	605,600
Stock options outstanding - Weighted - average exercise price
Beginning Balance	$ 25.19	$ 22.02	$ 20.04
Stock options granted	$ 36.66	$ 36.33	$ 22.10
Stock options exercised	$ 18.06	$ 13.64	$ 11.49
Stock options forfeited	$ 27.84	$ 32.07	$ 20.62
Ending Balance	$ 26.52	$ 25.19	$ 22.02