Summary of Repurchase Activity under Repurchase Programs (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Stock Repurchase Program [Line Items]
Repurchases paid, shares	944,398	756,143	773,258
Repurchases unsettled, shares	7,836	13,983	6,282
Prior year authorizations at January 1,	$ 13,284,000	$ 14,433,000	$ 14,793,000
Authorizations	50,000,000	14,410,000	12,164,000
Repurchases paid	(22,530,000)	(15,264,000)	(12,370,000)
Repurchases unsettled	(220,000)	(295,000)	(154,000)
Authorized dollars remaining as of December 31,	$ 40,534,000	$ 13,284,000	$ 14,433,000