Income Taxes - Summary of Differences between Income Tax Expense (Benefit) at the Statutory Federal Income Tax Rate and Total Income Tax Expense (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal income tax expense at statutory rates									$ 8,570	$ 7,934	$ 11,308
State income tax, net of federal income tax benefit									261	134	550
Insurance income									(265)	(176)	(371)
Effect of tax-exempt interest									(6,737)	(6,510)	(8,683)
Net tax credit									(292)	(349)	(341)
Valuation allowance reversal									(108)
Sequestration (reversal) accrual									(438)	438
Deferred tax remeasurement											8,448
Other									119	97	47
Provision for income taxes	$ 526	$ 435	$ 267	$ (118)	$ 309	$ 444	$ 314	$ 501	$ 1,110	$ 1,568	$ 10,958
Effective tax rate									2.72%	4.15%	32.95%