Stock Option Plan - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012 Y	Dec. 31, 2011	Dec. 31, 2010
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares of stock for issuance under the plan	1,920,000
Options exercisable period after, years	two
Percentage of options granted	20.00%
Duration of options granted, years	10
Stock options outstanding, exercise price range, lower	$ 15.40
Stock options outstanding, exercise price range, upper	$ 33.55
Weighted-average assumptions, risk-free interest rate		1.34%
Expected dividend yield		2.88%
Expected life		5 years 11 months 5 days
Expected volatility		26.92%
Weighted-average grant-date fair value of options granted	$ 0.00	$ 5.94	$ 0.00
Total intrinsic value of options exercised	$ 510,000	$ 619,000	$ 778,000
Total unrecognized compensation cost related to nonvested share-based compensation arrangements granted under the Plan	1,118,000
Weighted-average period of unrecognized compensation cost recognition, years	1 year 11 months 12 days
Total fair value of shares vested	423,000	369,000	213,000
Aggregate intrinsic value of vested stock options	$ 2,203,000