Investment Securities - Narrative (Details)	3 Months Ended		12 Months Ended
	Mar. 31, 2022 USD ($) security	Mar. 31, 2021 USD ($)	Dec. 31, 2012 shares	Dec. 31, 2021 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Cost of third party management	$ 300,000	$ 300,000
Number of securities with unrealized losses | security	4			1
SEREF
Debt Securities, Available-for-sale [Line Items]
Number of shares acquired (in shares) | shares			9,140,000
Equity security, fair value	$ 11,600,000			$ 11,600,000
Ownership percentage	2.00%
VIE eliminations
Debt Securities, Available-for-sale [Line Items]
Fair value of investment securities before consolidation of VIEs eliminated against VIE liabilities	$ 21,900,000
Fair value option
Debt Securities, Available-for-sale [Line Items]
Fair value of mortgage backed securities	1,600,000,000
RMBS
Debt Securities, Available-for-sale [Line Items]
Portion of securities with variable rate	$ 118,100,000
RMBS | Available-for-sale
Debt Securities, Available-for-sale [Line Items]
Portion of securities with variable rate (as a percent)	88.00%
RMBS | Fair value option
Debt Securities, Available-for-sale [Line Items]
Portion of securities with variable rate	$ 0
Fair value of investment securities before consolidation of VIEs	311,300,000
Unpaid principal balance of investment securities before consolidation of VIEs	179,900,000
CMBS | Fair value option
Debt Securities, Available-for-sale [Line Items]
Portion of securities with variable rate	98,200,000
Fair value of investment securities before consolidation of VIEs	1,300,000,000
Unpaid principal balance of investment securities before consolidation of VIEs	$ 2,800,000,000