Share Repurchases - Additional Information (Detail) (USD $) In Millions, except Share data in Thousands, unless otherwise specified	12 Months Ended		3 Months Ended	6 Months Ended	24 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Jun. 30, 2013 Share Repurchase Program [Member]	Jun. 30, 2013 Share Repurchase Program [Member]	Jun. 30, 2013 Share Repurchase Program [Member]
Shares Repurchased [Line Items]
Purchase of common stock	$ 200.0	$ 150.0
Repurchase of shares			600	1,200	4,900
Prepaid variable agreement cost			$ 31.3	$ 62.5	$ 187.5
Repurchase of shares average price per share			$ 53.49	$ 52.21	$ 38.23