Share-Based Compensation - Additional Information (Detail) (USD $)		1 Months Ended	12 Months Ended		1 Months Ended		12 Months Ended
	Dec. 31, 2012	May 31, 2012 Directors Stock Compensation Plan [Member]	Dec. 31, 2012 Directors Stock Compensation Plan [Member]	Dec. 31, 2011 Directors Stock Compensation Plan [Member]	Jul. 31, 2012 Performance Incentive Plan [Member]	Feb. 29, 2012 Performance Incentive Plan [Member]	Dec. 31, 2012 Performance Incentive Plan [Member]	Dec. 31, 2011 Performance Incentive Plan [Member]	Dec. 31, 2010 Performance Incentive Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Amortization of expense equally over a service period			1 year
Number of shares received by our non-employee directors		900
Weighted average grant-date fair value of awards granted			$ 41.06	$ 41.02			$ 39.62	$ 40.16	$ 29.38
Unrecognized compensation expense related to the DSCP awards			$ 148,000
Shares reserved for issuance	580,484		12,311				317,785
Granted awards, shares			10,800	11,104	4,800		35,706	41,664
Cash payment received by executive officer						181,500
Number of shares of common stock							21,600	24,000
Number of shares withheld							5,670	12,324	17,695
Payments of employees' tax obligations to taxing authorities							238,000	496,000	538,000
Intrinsic value of PIP awards							$ 1,200,000	$ 1,900,000	$ 2,700,000