Fair Value of Financial Instruments - Changes in the Level 3 Fair Value Category of Unobservable Inputs (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
State and Local Governments Taxable[Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	$ 1,490	$ 1,427
Change in Market Value	72	63
Purchases	0	0
Sales	0	0
Payments & Maturities	0	0
Ending Balance	1,562	1,490
State and Local Governments [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Beginning Balance	1,490	1,427
Change in Market Value	72	63
Purchases	0	0
Sales	0	0
Payments & Maturities	0	0
Ending Balance	$ 1,562	$ 1,490