Stock Repurchase Program - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended	3 Months Ended	1 Months Ended	9 Months Ended
	Feb. 22, 2011	Mar. 31, 2012	Dec. 14, 2011 Stock Repurchase Program Additional Authorization	Sep. 30, 2012 Stock Repurchase Program Additional Authorization
Stock Repurchase Program [Line Items]
Common stock repurchase authorized amount	$ 100.0		$ 150.0
Total stock repurchased, shares		3,996,118
Common stock repurchased and retired, shares		35,563		1,369,196
Common stock repurchased and retired, value		1.0		40.5
Common stock repurchase price per share		$ 29.40		$ 29.56
Common stock repurchase authorization amount available				$ 109.5