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		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
States and political subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Transfers out of Level 3				$ (4,000)
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	$ 7,185	$ 7,836	$ 8,222	8,495
Accretion of discount	4	3	11	9
Sales/payments received	(16)	(14)	(49)	(46)
Increase (decrease) in fair value of securities	354	(130)	(657)	(763)
Transfers out of Level 3				0
Balance at end of period	7,527	7,695	7,527	7,695
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	0	40	40	4,108
Accretion of discount	0	0	0	0
Sales/payments received	0	0	(40)	(35)
Increase (decrease) in fair value of securities	0	0	0	0
Transfers out of Level 3				(4,033)
Balance at end of period	$ 0	$ 40	$ 0	$ 40