Fair Value Measurements - Changes in Level 3 Securities (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 156,997	$ 96,886
Purchases	155,742	113,179
Maturities and calls	(100,282)	(57,476)
Amortization	(97)	(11)
Changes in fair value	478
Ending Balance	212,838	152,578
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	155,742	113,179
Maturities and calls	(100,245)	(57,411)
Amortization	(97)	(11)
Ending Balance	208,540	148,721
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