Stock Repurchase Program (Detail) - USD ($)	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Sep. 11, 2017	Nov. 09, 2015
Stock Repurchase Program [Line Items]
Repurchase of common stock, authorized amount			$ 125,000,000	$ 150,000,000
Aggregate shares repurchased (in shares)	959,786	748,448
Aggregate purchase price	$ 28,300,000	$ 34,500,000
Remaining available authorized repurchase amount	$ 81,100,000
Net Share Settlement
Stock Repurchase Program [Line Items]
Aggregate shares repurchased (in shares)	25,768	146,302
Value of shares surrendered to satisfy tax withholding obligations	$ 700,000	$ 6,800,000