Fair Value Measurements - Changes in the Fair Value of Securities Classified as Level 3 (Details) - USD ($)	3 Months Ended	9 Months Ended	12 Months Ended
	Mar. 31, 2024	Sep. 30, 2024	Dec. 31, 2023
Fair Value, Assets Measured on Recurring Basis [Roll Forward]
Transfers into Level 3		$ 0	$ 0
Transfers out of Level 3		$ 0	0
Gains (losses) included in net income
Fair Value, Asset, Recurring Basis, Unobservable Input Reconciliation, Asset, Gain (Loss), Statement of Other Comprehensive Income or Comprehensive Income [Extensible Enumeration]	Net unrealized gains (losses) on financial instruments at fair value	Net unrealized gains (losses) on financial instruments at fair value
Securitized debt at fair value, collateralized by loans held for investment | Variable Interest Entities, Primary Beneficiary | Level 3
Gains (losses) included in net income
Total unrealized (gains) losses for the period		$ 0	0
Fair Value, Liabilities Measured on Recurring Basis [Roll Forward]
Beginning balance Level 3	$ 7,601,881,000	7,601,881,000	7,100,742,000
Transfers into Level 3		0	0
Transfers out of Level 3		0	0
Issuance of debt		340,096,000	2,186,058,000
Principal payments		(874,437,000)	(1,222,593,000)
Sales and Settlements		0	(544,693,000)
Net (accretion) amortization		13,015,000	20,309,000
(Gains) losses included in net income		0	0
Other than temporary credit impairment losses		0	0
Realized (gains) losses on sales and settlements		0	(6,348,000)
Net unrealized (gains) losses included in income		233,855,000	68,406,000
(Gains) losses included in other comprehensive income		0	0
Total unrealized (gains) losses for the period		0	0
Securitized Debt, Ending balance, Level 3		7,314,411,000	7,601,881,000
Non-Agency RMBS | Level 3
Fair Value, Assets Measured on Recurring Basis [Roll Forward]
Beginning balance Level 3	1,043,806,000	1,043,806,000	1,147,481,000
Transfers into Level 3		0	0
Transfers out of Level 3		0	0
Purchases of assets		97,261,000	5,669,000
Principal payments		(57,893,000)	(83,730,000)
Sales and settlements		0	0
Net (accretion) amortization		26,607,000	38,789,000
Gains (losses) included in net income
(Increase) decrease in provision for credit losses		(5,389,000)	(11,371,000)
Realized gains (losses) on sales and settlements		0	13,000
Net unrealized (gains) losses included in income		19,444,000	(6,561,000)
Gains (losses) included in other comprehensive income		0	0
Total unrealized (gains) losses for the period		(2,000,000)	(46,484,000)
Ending balance Level 3		1,121,836,000	1,043,806,000
Fair Value, Liabilities Measured on Recurring Basis [Roll Forward]
Total unrealized (gains) losses for the period		(2,000,000)	(46,484,000)
Loans held for investment | Variable Interest Entities, Primary Beneficiary | Level 3
Fair Value, Assets Measured on Recurring Basis [Roll Forward]
Beginning balance Level 3	$ 11,125,052,000	11,125,052,000	11,154,600,000
Transfers into Level 3		0	0
Transfers out of Level 3		0	0
Purchases of assets		441,564,000	1,183,663,000
Principal payments		(880,620,000)	(1,247,364,000)
Sales and settlements		(2,959,000)	2,166,000
Net (accretion) amortization		(37,096,000)	(52,595,000)
Gains (losses) included in net income
(Increase) decrease in provision for credit losses		0	0
Realized gains (losses) on sales and settlements		0	0
Net unrealized (gains) losses included in income		404,605,000	84,582,000
Gains (losses) included in other comprehensive income		0	0
Total unrealized (gains) losses for the period		0	0
Ending balance Level 3		11,050,546,000	11,125,052,000
Fair Value, Liabilities Measured on Recurring Basis [Roll Forward]
Total unrealized (gains) losses for the period		$ 0	$ 0