Fair Value of Options Granted Calculated Using Weighted Average Assumptions (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted		58,550	46,950	58,550
Weighted average exercise price		$ 47.89	$ 41.27	$ 47.89
Weighted-average grant date fair value		$ 22.30	$ 19.08	$ 22.30
Assumptions:
Expected volatility		45.15%	47.70%	45.15%
Expected term (in years)		6 years	5 years 10 months 24 days	6 years
Risk-free interest rate		2.64%	1.43%	2.64%
Expected dividend yield