Fair Value Measurements - Changes in Level 3 Securities (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 156,997	$ 96,886
Purchases	181,752	166,339
Maturities and calls	(159,153)	(116,054)
Amortization	(163)	(35)
Changes in fair value	478
Ending Balance	179,911	147,136
Auction Rate Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	3,820	3,820
Changes in fair value	478
Ending Balance	4,298	3,820
Obligations Issued by States and Political Subdivisions [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	153,140	92,964
Purchases	181,752	166,339
Maturities and calls	(159,116)	(115,989)
Amortization	(163)	(35)
Ending Balance	175,613	143,279
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	37	102
Maturities and calls	$ (37)	(65)
Ending Balance		$ 37