Investment Securities - Proceeds from Sale, Gross Gains (Losses) Realized on Sales, Maturities and Other-Than-Temporary Impairment Charges Related to Securities Available for Sale (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Gain (Loss) on Securities [Line Items]
Proceeds from sale	$ 0	$ 948	$ 15,939
Net gains (losses) and impairment	70	22	8,102
Calls And Maturities [Member]
Gain (Loss) on Securities [Line Items]
Gross losses	0	0	0
Gross gains	70	29	3,383
Total gain (losses) maturities and impairment	70	29	3,383
Sales [Member]
Gain (Loss) on Securities [Line Items]
Gross losses	0	(7)	0
Available-for-sale Securities, Gross Realized Losses	0	0	4,719
Total gain (losses) from sales transactions	$ 0	$ (7)	$ 4,719