Fair Value Measurements - Changes in Level 3 Securities (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 164,876	$ 156,997
Purchases	47,430	155,742
Maturities and calls	(114,144)	(100,282)
Amortization	(113)	(97)
Changes in fair value		478
Ending Balance	98,049	212,838
Auction Rate Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	4,298	3,820
Changes in fair value		478
Ending Balance	4,298	4,298
Obligations Issued by States and Political Subdivisions [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	160,578	153,140
Purchases	47,430	155,742
Maturities and calls	(114,144)	(100,245)
Amortization	(113)	(97)
Ending Balance	$ 93,751	208,540
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance		37
Maturities and calls		$ (37)