Investment Securities - Narrative (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2021 USD ($) security	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) security	Jun. 30, 2020 USD ($)	Dec. 31, 2012 shares	Dec. 31, 2020 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Cost of third party management	$ 300,000	$ 300,000	$ 600,000	$ 700,000
Number of securities with unrealized losses | security	1		1			2
SEREF
Debt Securities, Available-for-sale [Line Items]
Number of shares acquired (in shares) | shares					9,140,000
Equity security, fair value	$ 11,900,000		$ 11,900,000			$ 11,200,000
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	19,000,000.0		$ 19,000,000.0
Fair value option
Debt Securities, Available-for-sale [Line Items]
Mortgage-backed Securities Available-for-sale, Fair Value Disclosure	1,500,000,000		1,500,000,000
RMBS
Debt Securities, Available-for-sale [Line Items]
Portion of securities with variable rate	$ 134,700,000		$ 134,700,000
RMBS | Available-for-sale
Debt Securities, Available-for-sale [Line Items]
Portion of securities with variable rate (as a percent)	87.00%		87.00%
RMBS | Fair value option
Debt Securities, Available-for-sale [Line Items]
Portion of securities with variable rate	$ 0		$ 0
Fair value of investment securities before consolidation of VIEs	232,100,000		232,100,000
Unpaid principal balance of investment securities before consolidation of VIEs	143,800,000		143,800,000
CMBS | Fair value option
Debt Securities, Available-for-sale [Line Items]
Portion of securities with variable rate	98,200,000		98,200,000
Fair value of investment securities before consolidation of VIEs	1,200,000,000		1,200,000,000
Unpaid principal balance of investment securities before consolidation of VIEs	$ 2,900,000,000		$ 2,900,000,000