Schedule of Common Stock Repurchases (Detail) $ / shares in Units, $ in Millions	3 Months Ended
	Mar. 31, 2018 USD ($) $ / shares shares
Schedule Of Common Share Purchase [Line Items]
Shares repurchased | shares	35,072
Cost of shares repurchased (in millions) | $	$ 21.3
Average price per share repurchased | $ / shares	$ 606.40