Common Stock and Treasury Stock - Additional Information (Detail) (USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended	6 Months Ended	12 Months Ended	30 Months Ended			0 Months Ended	6 Months Ended
	Feb. 29, 2012	Jun. 30, 2014	Dec. 31, 2011	Jun. 30, 2014	Feb. 28, 2014	Jul. 31, 2013	Sep. 13, 2012 Maximum	Jun. 30, 2014 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				100.0	100.0
Stock authorized to purchase under stock repurchase program, shares							7,500,000
Repurchase of common stock, shares		3,578,427		37,108,467
Repurchase of common stock, value		70.0		395.8
Remaining value of shares authorized for purchase under the stock repurchase program								$ 138.3