Fair Value - Reconciliation of All Assets Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
States and political subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Transfers out of Level 3		$ (4,033)
Significant Unobservable Inputs (Level 3) | Pooled trust preferred securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$ 8,222	8,495
Accretion of discount	4	4
Sales/payments received	(17)	(15)
Decrease in fair value of securities	(787)	(361)
Balance at end of period	7,422	8,123
Significant Unobservable Inputs (Level 3) | States and political subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	40	4,108
Accretion of discount		0
Sales/payments received	(40)	(35)
Decrease in fair value of securities		0
Balance at end of period	$ 0	$ 40