Comprehensive Income and Accumulated Other Comprehensive Income - Other Comprehensive Income (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income Loss Net Of Tax [Abstract]
Unrealized gain (loss) on available-for-sale securities, Pre-Tax Amount	$ 12,074	$ 15,670	$ (5,760)
Reclassification adjustment for gains (losses) realized in net income, Pre-Tax Amount	(925)	841	390
Net unrealized gain loss, Pre-Tax Amount	11,149	16,511	(5,370)
Other comprehensive income (loss), Pre-Tax Amount	11,149	16,511	(5,370)
Unrealized gain loss on available-for-sale securities, Tax Expense (Benefit)	2,536	3,291	(1,209)
Reclassification adjustment for gains (losses) realized in net income, Tax Expense (Benefit)	(195)	177	82
Net unrealized gain loss, Tax Expense (Benefit)	2,341	3,468	(1,127)
Other comprehensive income (loss), Tax Expense (Benefit)	2,341	3,468	(1,127)
Unrealized gain (loss) on available-for-sale securities, Net-of-Tax Amount	9,538	12,379	(4,551)
Reclassification adjustment for gains (losses) realized in net income, Net-of-Tax Amount	(730)	664	308
Net unrealized gain loss, Net-of-Tax Amount	8,808	13,043	(4,243)
Other comprehensive income (loss), Net-of-Tax Amount	$ 8,808	$ 13,043	$ (4,243)