Common Stock Repurchase Programs - Additional Information1 (Detail) - Net Share Settlement [Member] - USD ($) shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Stock Repurchase Program [Line Items]
Shares surrendered to satisfy tax withholding obligations	0.1	0.2	0.2	0.5
Value of shares surrendered to satisfy tax withholding obligations	$ 6.0	$ 7.1	$ 10.5	$ 23.1