Stock Repurchase Plan - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended	12 Months Ended	87 Months Ended	1 Months Ended	3 Months Ended
	Feb. 29, 2012	Dec. 31, 2011	Mar. 31, 2013	Sep. 13, 2012 Maximum	Mar. 31, 2013 Maximum
Maximum stock authorized to purchase under stock repurchase program	$ 262.1	$ 210.0		$ 113.0
Increase in maximum stock authorized to purchase under stock repurchase program	52.1
Stock authorized to purchase under stock repurchase program, shares				2,500,000
Repurchase of common stock, shares			9,519,872
Repurchase of common stock, value			244.9
Remaining authorized shares for repurchase under the stock repurchase program					1,800,000
Remaining value of shares authorized for purchase under the stock repurchase program					$ 88.2