Share Repurchases - Additional Information (Detail) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	12 Months Ended		3 Months Ended	21 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Mar. 31, 2013 Share Repurchase Program [Member]	Mar. 31, 2013 Share Repurchase Program [Member]
Shares Repurchased [Line Items]
Purchase of common stock	$ 200.0	$ 150.0
Repurchase of shares			600	4,300
Prepaid variable agreement			$ 31.3	$ 156.3
Repurchase of shares average price per share			$ 50.98	$ 36.17