Fair Value Measurements - Changes in Level 3 Securities (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 88,728	$ 82,600
Purchases	970	20,416
Maturities/redemptions	(26,352)	(22,068)
Transfer to Level 2		(4,459)
Amortization	(14)	(24)
Ending Balance	63,332	76,465
Auction Rate Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance		4,459
Transfer to Level 2		(4,459)
Obligations Issued by States and Political Subdivisions [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	88,728	78,141
Purchases	970	20,416
Maturities/redemptions	(26,352)	(22,068)
Amortization	(14)	(24)
Ending Balance	$ 63,332	$ 76,465