Income Taxes - Additional Information (Details) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating Loss Carryforwards [Line Items]
Unrealized gains resulting in a deferred tax liability		$ 26,300,000	$ 5,700,000
Deferred tax liability from unrealized gains on certain investments in debt securities		5,890,000	1,210,000
Net deferred tax assets		23,629,000	16,457,000
Deferred tax assets		23,629,000	16,457,000
Accrual for income tax interest or penalties		0
Income tax benefit related to share-based compensation		100,000	800,000	$ 1,100,000
Amortization reflected as income expense related to affordable housing project investments		900,000	900,000	1,000,000.0
Affordable housing project tax credits		800,000	800,000	800,000
Affordable housing project tax benefits		200,000	200,000	200,000
Carrying value of affordable housing tax credits		16,400,000	7,400,000
Affordable housing tax credits, unfunded amounts		9,900,000	500,000
Unrecognized income tax benefits		0
Tax rate applied to income before income taxes		21,122,000	27,008,000	18,381,000
Income tax credit and deferred tax asset related to adoption of CECL		5,500,000
Income tax benefit from CARES act		375,000	0	$ 0
U.S. Federal
Operating Loss Carryforwards [Line Items]
Net deferred tax assets		18,000,000.0
Deferred tax assets		2,900,000
State
Operating Loss Carryforwards [Line Items]
Net deferred tax assets		5,600,000
Deferred tax assets		$ 1,300,000
Tax rate applied to income before income taxes	$ 1,100,000		$ (400,000)