Equity (Detail) - USD ($) shares in Millions	1 Months Ended	12 Months Ended
	Feb. 28, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jul. 01, 2015
Stock Repurchase [Line Items]
Repurchase of common stock			$ 5,389,000	$ 4,997,000
Common Stock [Member]
Stock Repurchase [Line Items]
Stock repurchase program, authorized amount (in shares)					$ 25,000,000.0
Repurchase of common stock (in shares)		0.7
Repurchase of common stock		$ 10,400,000
Remaining authorized repurchase amount		$ 14,600,000
Subsequent Event [Member]
Stock Repurchase [Line Items]
Individual ownership percent maximum	4.99%