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Debt Obligations, Credit and Repurchase Facilities (Details) - USD ($)				1 Months Ended		6 Months Ended
	Mar. 31, 2021	Feb. 28, 2021	Jan. 31, 2021	Mar. 31, 2021	Jan. 31, 2021	Jun. 30, 2021	May 31, 2021	Dec. 31, 2020
Debt Obligations
Debt Carrying Value						$ 2,015,188,000		$ 2,234,883,000
Collateral Carrying Value						$ 2,757,662,000		2,724,420,000
Weighted Average Note Rate (as a percent)						2.32%
Joint Repurchase Facility
Debt Obligations
Maximum borrowing capacity						$ 2,000,000,000.00
Committed amount						50,000,000.0
Matures in March 2021						$ 5.75
Maximum advance rate						80.00%
$400M repurchase facility
Debt Obligations
Line of Credit Facility, Interest Rate at Period End			1.75%		1.75%
Repurchase facility - securities \| B Piece bonds
Debt Obligations
Collateral Carrying Value						$ 53,100,000		58,500,000
$400 million credit facility
Debt Obligations
Debt Carrying Value						400,000,000.0
Collateral Carrying Value						725,000,000.0
$200M credit facility
Debt Obligations
Debt Carrying Value							$ 200,000,000.0
Collateral Carrying Value							$ 250,000,000.0
Structured Business
Debt Obligations
Debt Carrying Value						1,623,398,000		1,282,845,000
Collateral Carrying Value						$ 2,310,652,000		$ 1,755,825,000
Weighted Average Note Rate (as a percent)						2.48%
Weighted average note rate including certain fees and costs (as a percent)						2.73%		2.97%
Unamortized deferred finance costs						$ 4,000,000.0		$ 3,300,000
Leverage on loans and investment portfolio financed through credit facilities and repurchase agreements, excluding securities repurchase facility, working capital line of credit and security agreements used to finance leasehold and capital expenditure improvements at corporate office (as a percent)								69.00%
Structured Business \| $1.8B joint repurchase facility
Debt Obligations
Debt Carrying Value						660,685,000		$ 681,006,000
Collateral Carrying Value						$ 947,249,000		1,054,562,000
Weighted Average Note Rate (as a percent)						2.51%
Maximum borrowing capacity						$ 1,800,000,000		1,800,000,000
Structured Business \| $400M repurchase facility
Debt Obligations
Variable rate, spread (as a percent)					1.75%
LIBOR Floor rate			0.35%		0.35%
Maximum borrowing capacity			$ 400,000,000.0		$ 400,000,000.0
Decrease in interest rate (as a percent)			2.00%
Utilization of credit facility			$ 250,000,000.0		$ 250,000,000.0
Structured Business \| $400M repurchase facility \| Minimum
Debt Obligations
Variable rate, spread (as a percent)					2.00%
Structured Business \| $400M repurchase facility \| Maximum
Debt Obligations
Variable rate, spread (as a percent)					2.20%
Structured Business \| $725M repurchase facility
Debt Obligations
Debt Carrying Value						335,477,000		191,622,000
Collateral Carrying Value						$ 466,109,000		259,559,000
Weighted Average Note Rate (as a percent)						2.10%
Maximum borrowing capacity						$ 725,000,000		725,000,000
Structured Business \| $200 million repurchase facility
Debt Obligations
Maximum borrowing capacity	$ 200,000,000.0			$ 200,000,000.0
Structured Business \| $200 million repurchase facility \| LIBOR
Debt Obligations
Maximum advance rate				80.00%
Structured Business \| $200 million repurchase facility \| Minimum \| LIBOR
Debt Obligations
Fixed interest rate (as a percent)	1.75%			1.75%
Structured Business \| $200 million repurchase facility \| Maximum \| LIBOR
Debt Obligations
Fixed interest rate (as a percent)	2.75%			2.75%
Structured Business \| $250M repurchase facility
Debt Obligations
Debt Carrying Value						104,251,000
Collateral Carrying Value						$ 260,922,000
Weighted Average Note Rate (as a percent)						1.87%
Maximum borrowing capacity						$ 250,000,000		250,000,000
Structured Business \| $198.7M repurchase facility
Debt Obligations
Debt Carrying Value						139,441,000		71,627,000
Collateral Carrying Value						$ 164,583,000		87,242,000
Weighted Average Note Rate (as a percent)						3.04%
Maximum borrowing capacity						$ 198,700,000		198,700,000
Structured Business \| $187.3M loan specific credit facilities
Debt Obligations
Debt Carrying Value						187,059,000		148,615,000
Collateral Carrying Value						$ 251,550,000		198,550,000
Weighted Average Note Rate (as a percent)						3.04%
Maximum borrowing capacity						$ 187,300,000		187,300,000
Structured Business \| $150M credit facility
Debt Obligations
Debt Carrying Value						8,576,000		23,606,000
Collateral Carrying Value	$ 150,000,000.0			$ 150,000,000.0		$ 15,132,000		31,809,000
Weighted Average Note Rate (as a percent)						3.90%
LIBOR Floor rate	0.15%
Maximum borrowing capacity						$ 150,000,000		150,000,000
Structured Business \| $100 million credit facility
Debt Obligations
Maximum borrowing capacity						100,000,000		100,000,000
Structured Business \| $100 million credit facility - one
Debt Obligations
Debt Carrying Value						58,625,000		39,346,000
Collateral Carrying Value						$ 83,800,000		47,912,000
Weighted Average Note Rate (as a percent)						1.88%
Maximum borrowing capacity						$ 100,000,000		100,000,000
Structured Business \| $100 million credit facility - two
Debt Obligations
Debt Carrying Value						9,936,000
Collateral Carrying Value						$ 13,773,000
Weighted Average Note Rate (as a percent)						4.06%
Structured Business \| $100 million repurchase facility
Debt Obligations
Debt Carrying Value						$ 48,847,000		31,780,000
Collateral Carrying Value						$ 61,211,000		40,551,000
Weighted Average Note Rate (as a percent)						1.88%
Maximum borrowing capacity						$ 100,000,000		100,000,000
Structured Business \| $50 million credit facility - one
Debt Obligations
Debt Carrying Value						22,407,000		15,992,000
Collateral Carrying Value						$ 28,009,000		21,300,000
Weighted Average Note Rate (as a percent)						2.13%
Maximum borrowing capacity						$ 50,000,000		50,000,000
Structured Business \| $30M working capital facility
Debt Obligations
Debt Carrying Value								30,000,000
Maximum borrowing capacity						30,000,000		30,000,000
Structured Business \| $50 million credit facility - two
Debt Obligations
Debt Carrying Value	$ 50,000,000.0			50,000,000.0
Structured Business \| $25 million credit facility
Debt Obligations
Debt Carrying Value						12,583,000		9,323,000
Collateral Carrying Value						$ 18,314,000		14,340,000
Weighted Average Note Rate (as a percent)						2.38%
Maximum borrowing capacity						$ 25,000,000		25,000,000
Structured Business \| $1M master security agreements
Debt Obligations
Debt Carrying Value						$ 944,000		1,441,000
Weighted Average Note Rate (as a percent)						4.01%
Maximum borrowing capacity						$ 1,000,000		1,000,000
Structured Business \| Repurchase facilities - securities (3)
Debt Obligations
Debt Carrying Value						$ 34,567,000		38,487,000
Weighted Average Note Rate (as a percent)						3.55%
Structured Business \| Repurchase facility - securities
Debt Obligations
Debt Carrying Value								10,000,000.0
Structured Business \| $21.6 million credit facility
Debt Obligations
Fixed interest rate (as a percent)		3.00%
Committed amount		$ 21,600,000
Structured Business \| 18.2 million credit facility
Debt Obligations
Maximum borrowing capacity	$ 18,200,000			$ 18,200,000
Structured Business \| 18.2 million credit facility \| Prime Rate
Debt Obligations
LIBOR Floor rate				3.25%
Agency Business
Debt Obligations
Debt Carrying Value						$ 391,790,000		952,038,000
Collateral Carrying Value						$ 447,010,000		968,595,000
Weighted Average Note Rate (as a percent)						1.64%
Unamortized deferred finance costs						$ 2,200,000		600,000
Agency Business \| $400M repurchase facility
Debt Obligations
Debt Carrying Value						119,382,000		174,515,000
Collateral Carrying Value						$ 119,398,000		174,555,000
Weighted Average Note Rate (as a percent)						1.60%
Maximum borrowing capacity						$ 400,000,000		400,000,000
Agency Business \| $150M credit facility
Debt Obligations
Debt Carrying Value						15,073,000		49,632,000
Collateral Carrying Value						$ 15,073,000		49,754,000
Weighted Average Note Rate (as a percent)						1.65%
Variable rate, spread (as a percent)	0.002%
LIBOR Floor rate	0.0025%
Maximum borrowing capacity	$ 150,000,000.0			$ 150,000,000.0		$ 150,000,000		150,000,000
Additional borrowing capacity	$ 200,000,000.0			$ 200,000,000.0
Agency Business \| $100 million credit facility
Debt Obligations
Debt Carrying Value						2,592,000		88,896,000
Collateral Carrying Value						$ 2,600,000		88,911,000
Weighted Average Note Rate (as a percent)						1.65%
Maximum borrowing capacity						$ 100,000,000		100,000,000
Agency Business \| $750M ASAP agreement
Debt Obligations
Debt Carrying Value						76,943,000		301,455,000
Collateral Carrying Value						$ 76,943,000		302,491,000
Weighted Average Note Rate (as a percent)						1.40%
Maximum borrowing capacity						$ 750,000,000		750,000,000
Agency Business \| $200M joint repurchase facility
Debt Obligations
Debt Carrying Value						104,876,000		42,808,000
Collateral Carrying Value						$ 135,934,000		56,186,000
Weighted Average Note Rate (as a percent)						1.85%
Maximum borrowing capacity						$ 200,000,000		200,000,000
Agency Business \| $200M credit facility
Debt Obligations
Debt Carrying Value						71,656,000		294,732,000
Collateral Carrying Value						$ 95,575,000		296,698,000
Weighted Average Note Rate (as a percent)						1.60%
Maximum borrowing capacity						$ 200,000,000		200,000,000
Agency Business \| $1.3 million repurchase facility
Debt Obligations
Debt Carrying Value						1,268,000
Collateral Carrying Value						$ 1,487,000
Weighted Average Note Rate (as a percent)						3.00%
Maximum borrowing capacity						$ 1,300,000		$ 1,300,000