Summary of Repurchase Activity under Repurchase Programs (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Stock Repurchase Program [Line Items]
Repurchases paid, shares	1,078,251	944,398	756,143
Repurchases unsettled, shares		7,836	13,983
Prior year authorizations at January 1,	$ 40,534,000	$ 13,284,000	$ 14,433,000
Authorizations	25,879,000	50,000,000	14,410,000
Repurchases paid	(27,028,000)	(22,530,000)	(15,264,000)
Repurchases unsettled		(220,000)	(295,000)
Authorized dollars remaining as of December 31,	$ 39,385,000	$ 40,534,000	$ 13,284,000