Income Taxes - Additional Information (Details) - USD ($)		3 Months Ended	12 Months Ended
	Dec. 22, 2017	Sep. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 21, 2017
Operating Loss Carryforwards [Line Items]
Unrealized losses on certain investment in debt securities			$ 4,658,000	$ 1,429,000
Unrealized losses on certain investments			17,700,000	5,600,000
Deferred income tax assets			28,954,000	25,417,000
Net deferred tax assets			28,954,000	25,417,000		$ 34,000,000
Accrual for income tax interest or penalties			0
Income tax benefit			(20,259,000)	(36,336,000)	$ (14,889,000)
Amortization reflected as income expense related to affordable housing project investments			1,000,000	700,000	39,000
Affordable housing project tax credits			800,000	600,000	32,000
Affordable housing project tax benefits			200,000	300,000	$ 67,000
Carrying value of affordable housing tax credits			8,300,000	9,300,000
Affordable housing tax credits, unfunded amounts			3,200,000	5,200,000
Unrecognized income tax benefits			0
One-time charge to income tax expense as a result of the tax cuts and jobs act of 2017	$ 8,600,000
Adjustment to true-up tax benefit related to tax cuts and jobs act of 2017			200,000
U.S. Federal
Operating Loss Carryforwards [Line Items]
Deferred income tax assets			4,700,000
Net deferred tax assets			20,500,000
State and Local Jurisdiction
Operating Loss Carryforwards [Line Items]
Deferred income tax assets			2,900,000
Net deferred tax assets			8,500,000
Accounting Standards Update 2016-09
Operating Loss Carryforwards [Line Items]
Income tax benefit		$ 800,000	$ 1,100,000	$ 1,100,000