Federal Funds Purchased and Securities Sold Under Agreement to Repurchase - Daily Securities Sold under Agreement to Repurchase (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amount Outstanding at End of Period	$ 30,200
Daily Securities Sold Under Agreement to Repurchase [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amount Outstanding at End of Period	$ 1,899	$ 1,814
Weighted Average Rate End of Period	1.08%	1.08%
Maximum Amount Borrowings Outstanding Month End	$ 2,568	$ 1,864
Approximate Average Outstanding in Period	$ 1,765	$ 1,302
Approximate Weighted Average Interest Rate For the Period	1.00%	0.65%
Term CDs Sold Under Agreement to Repurchase [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Amount Outstanding at End of Period	$ 28,340	$ 28,416
Weighted Average Rate End of Period	1.00%	1.39%
Maximum Amount Borrowings Outstanding Month End	$ 29,261	$ 28,416
Approximate Average Outstanding in Period	$ 28,379	$ 26,421
Approximate Weighted Average Interest Rate For the Period	2.52%	2.33%