Fair Value Measurements - Changes in Level 3 Securities (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 88,728	$ 82,600
Purchases	21,408	132,470
Maturities/redemptions	(96,812)	(121,753)
Transfer to Level 2		(4,459)
Amortization	(23)	(130)
Ending Balance	13,301	88,728
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	21,408	132,470
Maturities/redemptions	(96,812)	(121,753)
Amortization	(23)	(130)
Ending Balance	$ 13,301	$ 88,728