Summary of Stock Options Issued Pursuant to Stock Incentive Plans (Detail) (USD $)	9 Months Ended
	Sep. 30, 2011 Year
Number of Shares
Number of Shares Outstanding, Beginning Balance	3,510,538
Number of Shares, Granted	70,000
Number of Shares, Exercised	(232,499)
Number of Shares, Forfeited	(22,351)
Number of Shares Outstanding, Ending Balance	3,325,688
Number of Shares Exercisable, Ending Balance	2,267,032
Weighted Average Exercise Price
Weighted Average Exercise Price, Beginning Balance	$ 21.55
Weighted Average Exercise Price, Granted	$ 29.00
Weighted Average Exercise Price, Exercised	$ 11.95
Weighted Average Exercise Price, Forfeited	$ 20.19
Weighted Average Exercise Price, Ending Balance	$ 22.38
Weighted Average Exercise Price Exercisable, Ending Balance	$ 22.40
Weighted Average Remaining Contractual Term (Years)
Weighted Average Remaining Contractual Term (Years), Outstanding as of end of period	5.14
Weighted Average Remaining Contractual Term (Years), Exercisable as of end of period	4.55
Aggregate Intrinsic Value of In-the-Money Options
Aggregate Intrinsic Value of In-the-Money Options, Outstanding as of end of period	$ 21,022,107
Aggregate Intrinsic Value of In-the-Money Options, Exercisable as of end of period	$ 15,405,992