Summary of Repurchase Activity under Repurchase Programs (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stock Repurchase Program [Line Items]
Repurchases paid, shares	96	1,078	944
Repurchases unsettled, shares	3		8
Authorization remaining, beginning of period	$ 39,385,000	$ 40,534,000	$ 13,284,000
Authorizations		25,879,000	50,000,000
Repurchases paid	(4,335,000)	(27,028,000)	(22,530,000)
Repurchases unsettled	(158,000)		(220,000)
Authorization remaining, end of period	$ 34,892,000	$ 39,385,000	$ 40,534,000