Share Repurchases - Additional Information (Detail) (USD $)	1 Months Ended	23 Months Ended	3 Months Ended	9 Months Ended
	May 31, 2011	Sep. 30, 2011	Sep. 30, 2011 Repurchase Of Equity	Sep. 30, 2011 Repurchase Of Equity
Equity, Class of Treasury Stock [Line Items]
Share repurchase program, authorized amount	$ 50,000,000	$ 150,000,000
Number of shares repurchased		3,008,056	749,076	1,602,196
Aggregate purchase price		132,400,000	32,200,000	73,700,000
Weighted average price Per Share		$ 44.02	$ 43.03	$ 45.98
Amount available for future repurchases				$ 17,200,000