Securities Available for Sale - Allowance for Credit Losses (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, beginning	$ (592)	$ (15,571)	$ (18,736)	$ 0
Reversal of credit loss expense	496	3,669	3,288	(3,800)
Allowance arising from PCD financial assets		(5,640)	15,352	(13,742)
Allowance for credit losses, ending	(96)	(17,542)	(96)	(17,542)
CLUB Certificate asset-backed securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, beginning	(40)	(6,587)	(4,190)	0
Reversal of credit loss expense	40	2,087	236	(546)
Allowance arising from PCD financial assets		0	3,954	(3,954)
Allowance for credit losses, ending	0	(4,500)	0	(4,500)
Asset-backed subordinated securities
Debt Securities, Available-for-sale, Allowance for Credit Loss [Roll Forward]
Allowance for credit losses, beginning	(552)	(8,984)	(14,546)	0
Reversal of credit loss expense	456	1,582	3,052	(3,254)
Allowance arising from PCD financial assets		(5,640)	11,398	(9,788)
Allowance for credit losses, ending	$ (96)	$ (13,042)	$ (96)	$ (13,042)