DEBT OBLIGATIONS, NET - Additional Information (Details)											3 Months Ended	9 Months Ended	12 Months Ended
	Jul. 20, 2018	Apr. 03, 2018 USD ($)	Jan. 04, 2018	Dec. 06, 2017 USD ($)	Apr. 03, 2017 USD ($)	Apr. 01, 2017 USD ($)	Mar. 21, 2017 Extension option	Feb. 22, 2017 USD ($)	Dec. 17, 2014 USD ($)	Feb. 11, 2014 USD ($) Extension	Mar. 31, 2017 USD ($)	Sep. 30, 2018 agreement	Dec. 31, 2018 USD ($) agreement Extension counterparty	Dec. 31, 2017 USD ($) Extension	Dec. 31, 2016 USD ($) agreement	Jan. 01, 2021 USD ($)	Apr. 01, 2020 USD ($)	Dec. 27, 2018 USD ($)	May 07, 2018 USD ($)	Dec. 21, 2017 USD ($)	Oct. 17, 2017 USD ($)	Mar. 01, 2017 USD ($)	Feb. 19, 2016	Nov. 05, 2014 USD ($)	Sep. 19, 2012 USD ($)
Committed Loan and Securities Repurchase Facilities
Gross amounts of recognized liabilities													$ 663,686,000	$ 473,410,000
Mortgage loan receivables held for investment, net, at amortized cost:
Amortization of premiums													1,023,000	1,025,000	$ 894,000
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet													0	0
Proceeds from borrowings under debt obligations													$ 5,806,914,000	10,080,341,000	12,359,830,000
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Percent of FHLB advances to total debt obligations outstanding													28.90%
Retained earnings, appropriated													$ 849,300,000
Gain (loss) on extinguishment/defeasance of debt													(4,392,000)	(73,000)	$ 5,382,000
Tuebor
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Amount restricted from transfer													1,800,000,000
Revolving credit facility
Committed Loan and Securities Repurchase Facilities
Committed amount on credit agreement		$ 450,000,000											$ 350,000,000						$ 300,000,000
Debt borrowings term		1 year
Revolving credit facility | One-Month LIBOR
Committed Loan and Securities Repurchase Facilities
Committed amount on credit agreement										$ 266,430,000
Borrowing Under Credit Facilties and Debt Issuance Costs [Abstract]
Number of extension maturity periods | Extension										4
Length of extension options										12 months
Basis spread on variable rate (as a percent)	25.00%
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Stated interest rate on debt instrument (as a percent)										3.25%
Letters of credit
Committed Loan and Securities Repurchase Facilities
Committed amount on credit agreement										$ 25,000,000
Committed Loan Repurchase Facility
Committed Loan and Securities Repurchase Facilities
Number of agreements | agreement													6
Committed amount on credit agreement													$ 1,750,000,000
Committed financing													1,750,000,000	1,650,000,000				$ 100,000,000
Length of additional extension maturity periods			1 year
Gross amounts of recognized liabilities													497,531,000	398,653,000
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet													744,310,000	667,102,000
Term master repurchase agreement
Committed Loan and Securities Repurchase Facilities
Committed financing													400,000,000
Total Repurchase Facilities
Committed Loan and Securities Repurchase Facilities
Committed financing													$ 2,150,000,000	2,050,000,000
Repurchase agreements, number of counterparties | counterparty													8
Gross amounts of recognized liabilities													$ 663,685,000	473,410,000
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet													$ 932,113,000	753,424,000
Total Repurchase Facilities | Deutshe Bank, J.P. Morgan and Wells Fargo
Committed Loan and Securities Repurchase Facilities
Repurchase agreements, number of counterparties | counterparty													2
Excess collateral over amounts borrowed under repurchase agreements													$ 82,200,000
Ratio indebtedness over total equity (as a percent)													5.00%
Haircut on repurchase agreements (as a percent)													28.90%
Uncommitted securities Repurchase Facilities | Minimum
Borrowing Under Credit Facilties and Debt Issuance Costs [Abstract]
Advance rates (as a percent)													75.00%
Uncommitted securities Repurchase Facilities | Maximum
Borrowing Under Credit Facilties and Debt Issuance Costs [Abstract]
Advance rates (as a percent)													95.00%
Participation Financing - Mortgage Loan Receivable
Committed Loan and Securities Repurchase Facilities
Gross amounts of recognized liabilities													$ 2,500,000	3,100,000
Collateralized Debt Obligations [Abstract]
Proceeds from borrowings under debt obligations											$ 4,000,000
Senior Unsecured Notes
Collateralized Debt Obligations [Abstract]
Unamortized debt issuance costs													11,210,000
2/29/2020 | Committed Loan Repurchase Facility
Committed Loan and Securities Repurchase Facilities
Committed financing														$ 200,000,000
Number of additional extension maturity periods | Extension														2
Length of additional extension maturity periods														6 months
Gross amounts of recognized liabilities														$ 32,042,000
Borrowing Under Credit Facilties and Debt Issuance Costs [Abstract]
Number of extension maturity periods | Extension														1
Length of extension options														12 months
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet														$ 48,045,000
2/29/2020 | Committed Loan Repurchase Facility | Minimum
Collateralized Debt Obligations [Abstract]
Interest rate														4.25%
2/29/2020 | Committed Loan Repurchase Facility | Maximum
Collateralized Debt Obligations [Abstract]
Interest rate														4.50%
4/10/2018 | Committed Loan Repurchase Facility
Committed Loan and Securities Repurchase Facilities
Committed financing								$ 300,000,000.0						$ 300,000,000
Number of additional extension maturity periods | Extension							1							1
Length of additional extension maturity periods							364 days
Gross amounts of recognized liabilities														$ 63,007,000
Borrowing Under Credit Facilties and Debt Issuance Costs [Abstract]
Number of extension maturity periods | Extension														2
Length of extension options								1 year						364 days
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet														$ 125,379,000
4/10/2018 | Committed Loan Repurchase Facility | Bank
Committed Loan and Securities Repurchase Facilities
Number of additional extension maturity periods | Extension							1
Length of additional extension maturity periods							364 days
4/10/2018 | Committed Loan Repurchase Facility | Minimum
Collateralized Debt Obligations [Abstract]
Interest rate														3.73%
4/10/2018 | Committed Loan Repurchase Facility | Maximum
Collateralized Debt Obligations [Abstract]
Interest rate														4.73%
2/28/2022 | Revolving credit facility
Committed Loan and Securities Repurchase Facilities
Committed amount on credit agreement																						$ 200,000,000.0
10/1/2020 | Committed Loan Repurchase Facility
Committed Loan and Securities Repurchase Facilities
Committed financing													600,000,000	$ 600,000,000
Number of extension options | option							2
Length of additional extension maturity periods							12 months
Gross amounts of recognized liabilities													$ 180,597,000	$ 120,493,000
Borrowing Under Credit Facilties and Debt Issuance Costs [Abstract]
Number of extension maturity periods | Extension													2	2
Length of extension options													12 months	12 months
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet													$ 262,642,000	$ 160,031,000
10/1/2020 | Committed Loan Repurchase Facility | Minimum
Collateralized Debt Obligations [Abstract]
Interest rate													4.41%	3.23%
10/1/2020 | Committed Loan Repurchase Facility | Maximum
Collateralized Debt Obligations [Abstract]
Interest rate													4.96%	3.98%
Various Date | Uncommitted securities Repurchase Facilities
Committed Loan and Securities Repurchase Facilities
Gross amounts of recognized liabilities													$ 166,154,000	$ 74,757,000
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet													$ 187,803,000	$ 86,322,000
Various Date | Uncommitted securities Repurchase Facilities | Minimum
Collateralized Debt Obligations [Abstract]
Interest rate													2.99%	1.65%
Various Date | Uncommitted securities Repurchase Facilities | Maximum
Collateralized Debt Obligations [Abstract]
Interest rate													4.55%	3.31%
Various Date | Mortgage loan financing
Committed Loan and Securities Repurchase Facilities
Committed financing													$ 743,902,000	$ 692,696,000
Gross amounts of recognized liabilities													743,902,000	692,696,000
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet													$ 939,362,000	$ 911,034,000
Various Date | Mortgage loan financing | Minimum
Collateralized Debt Obligations [Abstract]
Interest rate													4.25%	4.25%
Various Date | Mortgage loan financing | Maximum
Collateralized Debt Obligations [Abstract]
Interest rate													7.00%	6.75%
Various Date | CLO Debt
Committed Loan and Securities Repurchase Facilities
Committed financing													$ 601,543,000	$ 688,479,000
Gross amounts of recognized liabilities													601,543,000	688,479,000
Collateralized Debt Obligations [Abstract]
Unamortized debt issuance costs													2,600,000	6,000,000
Gross amounts offset in the balance sheet													$ 710,502,000	$ 880,385,000						$ 431,500,000	$ 456,900,000
Various Date | CLO Debt | Minimum
Collateralized Debt Obligations [Abstract]
Interest rate													3.34%	2.36%
Various Date | CLO Debt | Maximum
Collateralized Debt Obligations [Abstract]
Interest rate													6.06%	5.08%
Various Date | CLO Debt | Weighted Average
Collateralized Debt Obligations [Abstract]
Interest rate													4.41%
Various Date | Senior Unsecured Notes
Collateralized Debt Obligations [Abstract]
Unamortized debt issuance costs													$ 11,200,000	$ 14,100,000
Debt Instrument, Face Amount													$ 1,166,201,000	$ 1,166,201,000
Various Date | Senior Unsecured Notes | Minimum
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Stated interest rate on debt instrument (as a percent)													5.25%	5.25%
Various Date | Senior Unsecured Notes | Maximum
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Stated interest rate on debt instrument (as a percent)													5.875%	5.875%
4/7/2019 | Committed Loan Repurchase Facility
Committed Loan and Securities Repurchase Facilities
Committed financing													$ 300,000,000
Number of additional extension maturity periods | Extension													1
Gross amounts of recognized liabilities													$ 120,631,000
Borrowing Under Credit Facilties and Debt Issuance Costs [Abstract]
Number of extension maturity periods | Extension													1
Length of extension options													364 days
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet													$ 204,747,000
4/7/2019 | Committed Loan Repurchase Facility | Minimum
Collateralized Debt Obligations [Abstract]
Interest rate													4.46%
4/7/2019 | Committed Loan Repurchase Facility | Maximum
Collateralized Debt Obligations [Abstract]
Interest rate													4.96%
5/6/2021 | Committed Loan Repurchase Facility
Committed Loan and Securities Repurchase Facilities
Committed financing													$ 300,000,000
Number of additional extension maturity periods | Extension													2
Length of additional extension maturity periods													6 months
Gross amounts of recognized liabilities													$ 79,886,000
Borrowing Under Credit Facilties and Debt Issuance Costs [Abstract]
Number of extension maturity periods | Extension													1
Length of extension options													12 months
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet													$ 117,382,000
5/6/2021 | Committed Loan Repurchase Facility | Minimum
Collateralized Debt Obligations [Abstract]
Interest rate													4.44%
5/6/2021 | Committed Loan Repurchase Facility | Maximum
Collateralized Debt Obligations [Abstract]
Interest rate													4.94%
Credit Agreement and Revolving Credit Facility
Borrowing Under Credit Facilties and Debt Issuance Costs [Abstract]
Unamortized debt issuance costs													$ 6,300,000	$ 7,800,000
Mortgage loan financing
Committed Loan and Securities Repurchase Facilities
Number of agreements | agreement												28	12		51
Mortgage loan receivables held for investment, net, at amortized cost:
Mortgage loan financing													$ 743,900,000	692,700,000
Net unamortized premiums													5,800,000	6,600,000
Amortization of premiums													1,000,000	1,000,000	$ 900,000
Pledged assets, real estate and lease intangibles, net													$ 939,400,000	$ 911,000,000
Mortgage loan financing | Minimum
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Stated interest rate on debt instrument (as a percent)													4.25%
Mortgage loan financing | Maximum
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Stated interest rate on debt instrument (as a percent)													7.00%
Borrowings from the Federal Home Loan Bank | Tuebor
Committed Loan and Securities Repurchase Facilities
Debt borrowings term													5 years 9 months	6 years
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Maximum advance limit				$ 2,000,000,000
Advance rates of total assets (as a percent)				40.00%
Advance rates of total equity (as a percent)				150.00%
FHLB borrowings outstanding													$ 1,300,000,000	$ 1,400,000,000
Additional committed term financing available from FHLB													$ 647,500,000	$ 630,000,000
Weighted average term													2 years 5 months 15 days	2 years 5 months 26 days
Weighted average interest rate													2.55%	1.61%
Maximum percent of FHLB advances to total assets																							40.00%
Borrowings from the Federal Home Loan Bank | Tuebor | Minimum
Borrowing Under Credit Facilties and Debt Issuance Costs [Abstract]
Advance rates (as a percent)													56.40%	49.60%
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Stated interest rate on debt instrument (as a percent)													1.18%	0.87%
Borrowings from the Federal Home Loan Bank | Tuebor | Maximum
Borrowing Under Credit Facilties and Debt Issuance Costs [Abstract]
Advance rates (as a percent)													95.20%	100.00%
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Stated interest rate on debt instrument (as a percent)													3.01%	2.74%
Borrowings from the Federal Home Loan Bank | Tuebor | CMBS and U.S. Agency Securities
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Collateral for debt instrument													$ 1,000,000,000	$ 861,700,000
Borrowings from the Federal Home Loan Bank | Tuebor | First mortgage commercial real estate loans
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Collateral for debt instrument													$ 637,200,000	915,900,000
Senior Unsecured Notes | Senior Notes Due, 2017
Collateralized Debt Obligations [Abstract]
Unamortized debt issuance costs					$ 22,800									200,000
Debt Instrument, Face Amount																									$ 325,000,000
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Stated interest rate on debt instrument (as a percent)																									7.375%
Debt instrument, minimum number of days to give notice for redemption without penalty													30 days
Debt instrument, maximum number of days to give notice for redemption without penalty													60 days
Debt instrument, repurchase price amount					297,700,000				$ 5,600,000						21,400,000									$ 325,000,000
Gain (loss) on extinguishment/defeasance of debt					(53,500)				(200,000)						300,000
Debt instrument, repurchased face amount					$ 297,700,000				$ 5,400,000						$ 21,900,000
Debt instrument, redemption price percentage						100.00%
Proceeds from Issuance of Senior Long-term Debt						$ 500,000,000
Affiliated Entity | Various Date | CLO Debt
Collateralized Debt Obligations [Abstract]
Ownership percentage																				25.00%	18.50%
Scenario, Forecast | Borrowings from the Federal Home Loan Bank | Tuebor
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Maximum advance limit																$ 750,000,000	$ 1,500,000,000
B Participation Interest | Affiliated Entity | Participation Financing - Mortgage Loan Receivable | Related Reserve IV LLC
Collateralized Debt Obligations [Abstract]
Interest expense, debt													$ 500,000	$ 500,000
LCFH
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Ownership interest in LCFH													88.80%
LCFH | LCFC
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Ownership interest in LCFH													100.00%