Fair Value Measurements - Changes in Level 3 Securities (Detail) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance		$ 82,600
Purchases		105,837
Maturities/redemptions		(72,640)
Transfer to Level 2		(4,459)
Amortization		(104)
Ending Balance		111,234
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	13,290	105,837
Maturities/redemptions	(78,196)	(72,640)
Amortization	(21)	(104)
Ending Balance	$ 23,801	$ 111,234