Summary of Stock Option Plan Activity (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding, beginning of period, Shares	2,892,882	2,715,430
Granted, Shares	535,300	517,500
Exercised, Shares	(213,645)	(33,224)
Expired, Shares		(158,850)
Outstanding, end of period, Shares	3,214,537	3,040,856
Outstanding, beginning of period, Weighted Average Exercise Price	$ 38.28	$ 38.57
Granted, Weighted Average Exercise Price	$ 42.49	$ 36.81
Exercised, Weighted Average Exercise Price	$ 30.26	$ 32.24
Expired, Weighted Average Exercise Price		$ 44.04
Outstanding, end of period, Weighted Average Exercise Price	$ 39.52	$ 38.06