Income Taxes - Additional Information (Details) - USD ($)		3 Months Ended	12 Months Ended
	Dec. 22, 2017	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating Loss Carryforwards [Line Items]
Unrealized gains resulting in a deferred tax liability			$ 5,700,000
Deferred tax liability from unrealized gains on certain investments in debt securities			1,210,000	$ 0
Unrealized losses resulting in a deferred tax asset				17,700,000
Deferred tax assets from unrealized losses on certain investments in debt securities			0	4,658,000
Net deferred tax assets			16,457,000	28,954,000
Deferred income tax assets			16,457,000	28,954,000
Accrual for income tax interest or penalties			0
Income tax benefit related to share-based compensation			(29,873,000)	(20,259,000)	$ (36,336,000)
Amortization reflected as income expense related to affordable housing project investments			900,000	1,000,000.0	700,000
Affordable housing project tax credits			800,000	800,000	600,000
Affordable housing project tax benefits			200,000	200,000	300,000
Carrying value of affordable housing tax credits			7,400,000	8,300,000
Affordable housing tax credits, unfunded amounts			500,000	3,200,000
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
Tax rate applied to income before income taxes			27,008,000	18,381,000	27,720,000
U.S. Federal
Operating Loss Carryforwards [Line Items]
Net deferred tax assets			12,900,000
Deferred income tax assets			3,600,000
State
Operating Loss Carryforwards [Line Items]
Net deferred tax assets			3,500,000
Deferred income tax assets			1,100,000
Tax rate applied to income before income taxes		$ 1,100,000	(400,000)
Accounting Standards Update 2016-09
Operating Loss Carryforwards [Line Items]
Income tax benefit related to share-based compensation			$ 800,000	$ 1,100,000	$ 1,100,000