Schedule of Common Stock Repurchases (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Schedule Of Common Share Purchase [Line Items]
Shares repurchased	113,100	58,950
Cost of shares repurchased (in millions)	$ 53.3	$ 26.6
Average price per share repurchased	$ 471.15	$ 451.77