Investments - Available-for-sale (Details) - USD ($) $ in Thousands		Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Gross unrealized gains		$ 3,600	$ 1,200
Gross unrealized losses		(200)	(800)
Interest receivable on securities		200	200
Debt securities
Debt Securities, Available-for-sale [Line Items]
Basis		336,664	273,459
Gross unrealized gains		3,621	1,152
Gross unrealized losses		(203)	(777)
Carrying value		340,082	273,834
Beneficial interests in securitization trusts
Debt Securities, Available-for-sale [Line Items]
Basis		139,494	91,418
Gross unrealized gains		0	0
Gross unrealized losses		0	0
Carrying value	[1]	139,494	91,418
Accumulated net investment gain (loss) attributable to noncontrolling interest
Debt Securities, Available-for-sale [Line Items]
Basis		476,158	364,877
Gross unrealized gains		3,621	1,152
Gross unrealized losses		(203)	(777)
Carrying value		$ 479,576	$ 365,252

[1] Investments in beneficial interests includes allowance for expected credit losses of $0.6 million and $4.5 million at September 30, 2021 and December 31, 2020, respectively.