Fair Value - Reconciliation of All Assets Measured at Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3) (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
State and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Transfers out of Level 3		$ (4,000)
Transfers into Level 3		4,100
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	$ 8,448
Accretion (amortization) of discount	15	12	17
Sales/payments received	(64)	(62)	(338)
Increase (decrease) in fair value of securities	(260)	(223)	368
Transfers out of Level 3		0
Transfers into Level 3			0
Balance at end of period	7,913	8,222	8,495
Significant Unobservable Inputs (Level 3) | State and Political Subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	40	4,108	0
Accretion (amortization) of discount	0	0	(56)
Sales/payments received	(40)	(35)	0
Increase (decrease) in fair value of securities	0	0	28
Transfers out of Level 3		(4,033)
Transfers into Level 3			4,136
Balance at end of period	$ 0	$ 40	$ 4,108