Schedule of Common Stock Repurchases (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Schedule Of Common Share Purchase [Line Items]
Shares repurchased	324,661	148,231	412,844	549,853
Cost of shares repurchased (in millions)	$ 152.0	$ 62.5	$ 192.5	$ 222.4
Average price per share repurchased	$ 468.11	$ 421.89	$ 466.33	$ 404.55