DEBT - Narrative Disclosures (Details)					1 Months Ended	7 Months Ended	9 Months Ended	12 Months Ended			29 Months Ended
	Dec. 13, 2018	Dec. 12, 2018	Dec. 21, 2017	Oct. 01, 2017	Dec. 31, 2017 USD ($)	Aug. 03, 2017	Sep. 30, 2019	Dec. 31, 2019 USD ($) extension	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2019 USD ($) extension	Nov. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	May 31, 2019 USD ($)
Funding agreements
Total Commitment								$ 1,471,424,000	$ 1,475,989,000		$ 1,471,424,000
Outstanding balance								894,744,000	887,974,000		894,744,000
Early extinguishment of debt costs								0	0	$ (768,000)
Amount of debt discount on the initial draw down amount								$ 2,600,000
Maximum
Funding agreements
Extension period of maturity date								12 months
Notes Payable to Banks
Funding agreements
Interest rate margin (as a percent)								3.75%
Outstanding balance												$ 23,500,000		$ 32,400,000
Notes Payable to Banks | NORTH CAROLINA
Funding agreements
Outstanding balance														$ 40,500,000
Notes Payable to Banks | NEW YORK
Funding agreements
Interest rate margin (as a percent)								3.00%
Outstanding balance								$ 28,300,000			28,300,000
Notes Payable to Banks | SOUTH CAROLINA
Funding agreements
Interest rate margin (as a percent)								2.50%
Outstanding balance								$ 27,900,000			27,900,000	$ 34,600,000
Secured term loan
Funding agreements
Total Commitment								$ 110,000,000	110,000,000		$ 110,000,000
Number of extension periods available for maturity date | extension								1			1
Extension period of maturity date								12 months
Outstanding balance								$ 110,000,000	$ 110,000,000		$ 110,000,000
Aggregate principal amount								$ 110,000,000			$ 110,000,000
Secured term loan | LIBOR
Funding agreements
Interest rate margin (as a percent)			7.20%					5.00%	6.00%	8.70%
Repayments of debt					$ 45,000,000
Early extinguishment of debt costs										$ 768,000
LIBOR floor (as a percent)								1.00%			1.00%
Percentage of accretion of original issue discount and associated costs								8.00%	7.60%		8.00%
Secured term loan | Minimum
Funding agreements
Tangible net worth (as a percent)								80.00%
Net proceeds (as a perccent)								80.00%
Percentage of tangible net worth required to be maintained (as a percent)								65.00%
Asset coverage ratio								110.00%
Unencumbered asset ratio								120.00%
Secured term loan | Maximum
Funding agreements
Debt Instrument Covenant Ratio of Debt to Tangible Net Worth								4
Secured Term Loan, Extension Period One | LIBOR
Funding agreements
Debt instrument, interest rate,								0.125%
Secured Term Loan, Extension Period Two | LIBOR
Funding agreements
Debt instrument, interest rate,								0.375%
Secured Term Loan, Extension Period Three | LIBOR
Funding agreements
Debt instrument, interest rate,								0.75%
U.S. Bank Facility
Funding agreements
Total Commitment								$ 185,989,000	$ 185,989,000		$ 185,989,000
Outstanding balance								43,045,000	148,475,000		43,045,000
U.S. Bank Facility | 30 day LIBOR
Funding agreements
Total Commitment								$ 186,000,000			$ 186,000,000
U.S. Bank Facility | Secured revolving funding facility
Funding agreements
Line of credit facility								0.25%
U.S. Bank Facility | Revolving master repurchase facility
Funding agreements
Number of extension periods available for maturity date | extension								2			2
Extension period of maturity date								12 months
U.S. Bank Facility | Revolving master repurchase facility | 30 day LIBOR
Funding agreements
Interest rate margin (as a percent)								50.00%
Non-utilization/commitment fee								$ 246,000	83,000
U.S. Bank Facility | Revolving master repurchase facility | One-month LIBOR
Funding agreements
Interest rate margin (as a percent)								2.25%
U.S. Bank Facility | Secured funding facility
Funding agreements
Non-utilization/commitment fee									0
Wells Fargo Facility
Funding agreements
Total Commitment								$ 500,000,000	500,000,000		$ 500,000,000
Outstanding balance								$ 360,354,000	274,071,000		$ 360,354,000
Wells Fargo Facility | Secured revolving funding facility
Funding agreements
Number of extension periods available for maturity date | extension								3			3
Extension period of maturity date								12 months
Non-utilization threshold (less than)								75.00%			75.00%
Non-utilization/commitment fee								$ 618,000	149,000	362,000
Line of credit facility								0.25%
Wells Fargo Facility | Secured revolving funding facility | Minimum
Funding agreements
Tangible net worth to be maintained								$ 135,500,000
Tangible net worth (as a percent)								80.00%
Net proceeds (as a perccent)								80.00%
Wells Fargo Facility | Secured revolving funding facility | Minimum | 30 day LIBOR
Funding agreements
Interest rate margin (as a percent)	1.75%							1.50%
Wells Fargo Facility | Secured revolving funding facility | Maximum
Funding agreements
Debt Instrument Covenant Ratio of Debt to Tangible Net Worth								4
Ratio of recourse debt to tangible net worth								3
Ratio of EBITDA to fixed charges								1.25
Wells Fargo Facility | Secured revolving funding facility | Maximum | 30 day LIBOR
Funding agreements
Interest rate margin (as a percent)	2.35%							2.25%
Citibank Facility
Funding agreements
Total Commitment								$ 325,000,000	325,000,000		$ 325,000,000
Outstanding balance								126,603,000	184,003,000		126,603,000
Citibank Facility | Secured revolving funding facility
Funding agreements
Total Commitment								$ 325,000,000			$ 325,000,000
Number of extension periods available for maturity date | extension								2			2
Extension period of maturity date								12 months
Non-utilization/commitment fee								$ 388,000	143,000	165,000
Liquidity to be maintained as a percentage								5.00%
Line of credit facility								0.25%
Citibank Facility | Secured revolving funding facility | Minimum
Funding agreements
Tangible net worth (as a percent)								80.00%
Net proceeds (as a perccent)								80.00%
Amount of liquidity to be maintained								$ 5,000,000
Citibank Facility | Secured revolving funding facility | Minimum | 30 day LIBOR
Funding agreements
Interest rate margin (as a percent)		2.25%						1.50%
Citibank Facility | Secured revolving funding facility | Maximum
Funding agreements
Debt Instrument Covenant Ratio of Debt to Tangible Net Worth								4
Ratio of recourse debt to tangible net worth								3
Ratio of EBITDA to fixed charges								1.25
Amount of liquidity to be maintained								$ 10,000,000
Citibank Facility | Secured revolving funding facility | Maximum | 30 day LIBOR
Funding agreements
Interest rate margin (as a percent)		2.50%						2.25%
BAML Facility
Funding agreements
Total Commitment								$ 36,280,000	125,000,000		$ 36,280,000
Outstanding balance								$ 36,280,000	36,280,000		36,280,000
Period immediately preceding								12 months
BAML Facility | Secured revolving funding facility
Funding agreements
Line of credit facility								0.125%
BAML Facility | Secured funding facility
Funding agreements
Total Commitment								$ 125,000,000			$ 125,000,000
Number of extension periods available for maturity date | extension								1			1
Extension period of maturity date								12 months
Non-utilization/commitment fee								$ 43,000	21,000	52,000
Term of debt								2 years
Facility used on average (as a percent)								50.00%
BAML Facility | Secured funding facility | One-month LIBOR
Funding agreements
Interest rate margin (as a percent)							2.00%
BAML Facility | Secured funding facility | Minimum
Funding agreements
Tangible net worth (as a percent)								80.00%
Net proceeds (as a perccent)								80.00%
BAML Facility | Secured funding facility | Minimum | One-month LIBOR
Funding agreements
Interest rate margin (as a percent)				2.25%
BAML Facility | Secured funding facility | Maximum
Funding agreements
Debt Instrument Covenant Ratio of Debt to Tangible Net Worth								4
Ratio of recourse debt to tangible net worth								3
Ratio of EBITDA to fixed charges								1.25
BAML Facility | Secured funding facility | Maximum | One-month LIBOR
Funding agreements
Interest rate margin (as a percent)				2.75%
CNB Facility
Funding agreements
Total Commitment													$ 50,000,000.0
CNB Facility | Amended City National Bank Facility
Funding agreements
Total Commitment													75,000,000.0
CNB Facility | CNB Facility
Funding agreements
Total Commitment								$ 50,000,000	50,000,000		$ 50,000,000		$ 50,000,000
Number of extension periods available for maturity date | extension								2			2
Extension period of maturity date								12 months
Non-utilization/commitment fee								$ 136,000	166,000	$ 184,000
Line of credit facility								0.375%
Outstanding balance								$ 30,500,000	0		$ 30,500,000
Period immediately preceding								12 months
CNB Facility | CNB Facility | One, two, three or six month LIBOR
Funding agreements
Interest rate margin (as a percent)								3.00%
CNB Facility | CNB Facility | Federal funds rate
Funding agreements
Interest rate margin (as a percent)								0.50%
CNB Facility | CNB Facility | One-month LIBOR
Funding agreements
Interest rate margin (as a percent)								1.00%
CNB Facility | CNB Facility | Base rate
Funding agreements
Interest rate margin (as a percent)								1.25%
CNB Facility | CNB Facility | Minimum
Funding agreements
Tangible net worth (as a percent)								80.00%
Net proceeds (as a perccent)								80.00%
Facility used on average (as a percent)								75.00%
CNB Facility | CNB Facility | Minimum | One, two, three or six month LIBOR
Funding agreements
Interest rate margin (as a percent)								2.65%
CNB Facility | CNB Facility | Maximum
Funding agreements
Debt Instrument Covenant Ratio of Debt to Tangible Net Worth								4.00
Ratio of recourse debt to tangible net worth								3
Ratio of EBITDA to fixed charges								1.25
MetLife Facility
Funding agreements
Total Commitment								$ 180,000,000	180,000,000		180,000,000
Outstanding balance								$ 131,807,000	135,145,000		$ 131,807,000
MetLife Facility | Secured revolving funding facility
Funding agreements
Non-utilization threshold (less than)								65.00%			65.00%
Non-utilization/commitment fee								$ 5,000	7,000
MetLife Facility | Revolving master repurchase facility
Funding agreements
Total Commitment								$ 180,000,000			$ 180,000,000
Number of extension periods available for maturity date | extension								2			2
Extension period of maturity date								12 months
Period immediately preceding								12 months
MetLife Facility | Revolving master repurchase facility | 30 day LIBOR
Funding agreements
Interest rate margin (as a percent)						2.35%					2.30%
Line of credit facility								0.25%
MetLife Facility | Revolving master repurchase facility | Minimum
Funding agreements
Tangible net worth (as a percent)								80.00%
MetLife Facility | Revolving master repurchase facility | Maximum
Funding agreements
Debt Instrument Covenant Ratio of Debt to Tangible Net Worth								4
Ratio of recourse debt to tangible net worth								3
Ratio of EBITDA to fixed charges								1.25
Notes Payable to Banks
Funding agreements
Total Commitment								$ 84,155,000	0		$ 84,155,000
Outstanding balance								56,155,000	$ 0		$ 56,155,000
Short-term debt, increase in borrowing limit								$ 30,000,000