Schedule of Information about Options Outstanding (Detail) (USD $)	3 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options outstanding, Shares	688,476	784,226
Granted, Shares	11,475	11,475
Forfeited, Shares		(650)
Expired, Shares	(250)	(250)
Exercised, Shares	(39,600)	(134,700)
Options outstanding, Shares	660,101	660,101
Options outstanding, Weighted Average Exercise Price	$ 20.69	$ 20.40
Granted, Weighted Average Exercise Price	$ 21.24	$ 21.24
Forfeited, Weighted Average Exercise Price		$ 19.65
Expired, Weighted Average Exercise Price	$ 22.00	$ 22.00
Exercised, Weighted Average Exercise Price	$ 20.45	$ 18.93
Options outstanding, Weighted Average Exercise Price	$ 20.72	$ 20.72
Options outstanding, beginning of period, Weighted Average Grant Date Fair Value	$ 4.69	$ 4.62
Granted, Weighted Average Grant Date Fair Value	$ 4.83	$ 4.83
Forfeited, Weighted Average Grant Date Fair Value		$ 4.62
Expired, Weighted Average Grant Date Fair Value	$ 4.90	$ 4.90
Exercised, Weighted Average Grant Date Fair Value	$ 4.64	$ 4.22
Options outstanding, end of period, Weighted Average Grant Date Fair Value	$ 4.70	$ 4.70