Summary of Stock Options Issued under Various Stock Incentive Plans (Detail) (USD $)	12 Months Ended
	Dec. 31, 2011 Year	Dec. 31, 2010	Dec. 31, 2009
Number of Shares
Beginning Balance	3,510,538	3,556,873	3,428,297
Granted	367,202	338,950	505,183
Exercised	(361,093)	(235,986)	(150,134)
Forfeited	(26,591)	(106,625)	(125,606)
Expired		(42,674)	(100,867)
Ending Balance	3,490,056	3,510,538	3,556,873
Number of Shares Exercisable, Ending Balance	2,289,018
Weighted Average Exercise Price
Beginning Balance	$ 21.55	$ 20.72	$ 21.69
Granted	$ 28.95	$ 24.48	$ 16.17
Exercised	$ 12.40	$ 13.34	$ 12.06
Forfeited	$ 20.53	$ 18.11	$ 31.98
Expired		$ 30.31	$ 29.82
Ending Balance	$ 23.28	$ 21.55	$ 20.72
Weighted Average Exercise Price Exercisable, Ending Balance	$ 22.88
Weighted Average Remaining Contractual Term (Years)
Weighted Average Remaining Contractual Term (Years), Outstanding as of end of year	5.37
Weighted Average Remaining Contractual Term (Years), Exercisable as of end of year	4.62
Aggregate Intrinsic Value of In-the-Money Options
Aggregate Intrinsic Value of In-the-Money Options, Outstanding as of end of year	$ 21,943,485
Aggregate Intrinsic Value of In-the-Money Options, Exercisable as of end of year	$ 16,297,256