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Securities Available for Sale - Amortized cost/fair value (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 268,732		$ 159,164
Gross Unrealized Gains	9,202		2,771
Gross Unrealized Losses	(2,908)		(973)
Allowance for Credit Losses	(96)	$ (592)	(18,736)	$ (17,542)	$ (15,571)	$ 0
Fair Value	274,930		142,226
Securities pledged as collateral at fair value	66,400		133,500
U.S. agency residential mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	115,691
Gross Unrealized Gains	80
Gross Unrealized Losses	(1,877)
Allowance for Credit Losses	0
Fair Value	113,894
Asset-backed senior securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	37,635		75,332
Gross Unrealized Gains	73		67
Gross Unrealized Losses	0		(27)
Allowance for Credit Losses	0		0
Fair Value	37,708		75,372
Securities pledged as collateral at fair value	248,900		133,500
Commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	28,963
Gross Unrealized Gains	48
Gross Unrealized Losses	(367)
Allowance for Credit Losses	0
Fair Value	28,644
Other asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	27,642
Gross Unrealized Gains	244
Gross Unrealized Losses	(40)
Allowance for Credit Losses	0
Fair Value	27,846		200
U.S. agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	27,943
Gross Unrealized Gains	15
Gross Unrealized Losses	(548)
Allowance for Credit Losses	0
Fair Value	27,410
CLUB Certificate asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	21,520		54,525
Gross Unrealized Gains	2,672		576
Gross Unrealized Losses	(16)		(772)
Allowance for Credit Losses	0	(40)	(4,190)	(4,500)	(6,587)	0
Fair Value	24,176		50,139
Asset-backed subordinated securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,836		29,107
Gross Unrealized Gains	6,070		2,128
Gross Unrealized Losses	0		(174)
Allowance for Credit Losses	(96)	$ (552)	(14,546)	$ (13,042)	$ (8,984)	$ 0
Fair Value	11,810		16,515
Municipal securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	3,302
Gross Unrealized Gains	0
Gross Unrealized Losses	(60)
Allowance for Credit Losses	0
Fair Value	3,242
Other securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	200		200
Gross Unrealized Gains	0		0
Gross Unrealized Losses	0		0
Allowance for Credit Losses	0		0
Fair Value	200		200
Asset-backed securities, securitized loans and receivables, subject to restrictions on transfer
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 39,000		$ 119,300