DEBT OBLIGATIONS, NET - Additional Information (Details)							12 Months Ended
	Dec. 06, 2017	Mar. 21, 2017 Extension	Feb. 22, 2017 USD ($)	Nov. 09, 2016	Apr. 19, 2016 Extension	Feb. 11, 2014 USD ($) Extension	Dec. 31, 2017 USD ($) counterparty agreement Extension	Dec. 31, 2016 USD ($) agreement Extension	Dec. 31, 2015 USD ($) agreement	Jan. 01, 2021 USD ($)	Apr. 01, 2020 USD ($)	Dec. 31, 2018 USD ($)	Dec. 21, 2017 USD ($)	Oct. 17, 2017 USD ($)	Mar. 23, 2017 Extension	Mar. 01, 2017 USD ($)	Aug. 03, 2016 USD ($) Extension	Jul. 01, 2016 USD ($)	Jun. 28, 2016 USD ($)	Feb. 19, 2016
Committed Loan and Securities Repurchase Facilities
Gross amounts of recognized liabilities							$ 473,410,000	$ 1,107,185,000
Mortgage loan receivables held for investment, net, at amortized cost:
Amortization of premiums							(1,025,000)	(894,000)	$ (902,000)
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet							0	0
Participation Financing - Mortgage Loan Receivable
Proceeds from borrowings under debt obligations							$ 10,080,341,000	12,359,830,000	$ 16,280,023,000
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Percent of FHLB advances to total debt obligations outstanding							31.30%
Retained earnings, appropriated							$ 780,000,000
Tuebor
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Amount restricted from transfer							$ 1,200,000,000
Revolving credit facility | One-Month LIBOR
Committed Loan and Securities Repurchase Facilities
Committed amount on credit agreement						$ 241,430,000
Number of extension maturity periods | Extension						3
Length of extension options						12 months
Number of additional extension maturity periods | Extension															2
Borrowing Under Credit Facilties and Debt Issuance Costs [Abstract]
Basis spread on variable rate (as a percent)						3.50%
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							5
Committed amount on credit agreement							$ 1,650,000,000
Committed amount on master repurchase agreement							1,650,000,000	1,650,000,000
Gross amounts of recognized liabilities							398,653,000	567,163,000
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet							667,102,000	961,791,000
Term master repurchase agreement
Committed Loan and Securities Repurchase Facilities
Committed amount on master repurchase agreement							400,000,000
Total Repurchase Facilities
Committed Loan and Securities Repurchase Facilities
Committed amount on master repurchase agreement							$ 2,050,000,000	2,050,000,000
Repurchase agreements, number of counterparties | counterparty							7
Gross amounts of recognized liabilities							$ 473,410,000	1,107,185,000
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet							$ 753,424,000	1,602,831,000
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							$ 74,400,000
Ratio indebtedness over total equity (as a percent)							5.00%
Haircut on repurchase agreements (as a percent)							37.30%
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
Participation Financing - Mortgage Loan Receivable
Proceeds from borrowings under debt obligations							$ 4,000,000
Interest expense							500,000
5/24/2018 | Committed Loan Repurchase Facility
Committed Loan and Securities Repurchase Facilities
Committed amount on master repurchase agreement							$ 450,000,000
Debt borrowings term					1 year
Number of extension maturity periods | Extension					2		3
Length of extension options							12 months
Gross amounts of recognized liabilities							$ 183,111,000
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet							$ 333,647,000
5/24/2018 | Committed Loan Repurchase Facility | Minimum
Collateralized Debt Obligations [Abstract]
Interest rate							3.63%
5/24/2018 | Committed Loan Repurchase Facility | Maximum
Collateralized Debt Obligations [Abstract]
Interest rate							4.48%
9/30/2018 | Term master repurchase agreement
Committed Loan and Securities Repurchase Facilities
Committed amount on master repurchase agreement								400,000,000										$ 400,000,000
Gross amounts of recognized liabilities								228,317,000
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet								$ 272,402,000
9/30/2018 | Term master repurchase agreement | Minimum
Collateralized Debt Obligations [Abstract]
Interest rate								1.00%
9/30/2018 | Term master repurchase agreement | Maximum
Collateralized Debt Obligations [Abstract]
Interest rate								2.59%
6/28/2019 | Committed Loan Repurchase Facility
Committed Loan and Securities Repurchase Facilities
Committed amount on master repurchase agreement							$ 100,000,000	$ 100,000,000											$ 100,000,000
Gross amounts of recognized liabilities							0	27,132,000
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet							0	$ 36,166,000
6/28/2019 | Committed Loan Repurchase Facility | Minimum
Collateralized Debt Obligations [Abstract]
Interest rate								2.90%
6/28/2019 | Committed Loan Repurchase Facility | Maximum
Collateralized Debt Obligations [Abstract]
Interest rate								3.13%
8/2/2019 | Committed Loan Repurchase Facility
Committed Loan and Securities Repurchase Facilities
Committed amount on master repurchase agreement								$ 100,000,000									$ 100,000,000
Number of extension maturity periods | Extension								1									1
Length of extension options								12 months
Number of additional extension maturity periods | Extension								2									2
Length of additional extension maturity periods								6 months
Gross amounts of recognized liabilities								$ 71,290,000
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet								$ 110,271,000
8/2/2019 | Committed Loan Repurchase Facility | Minimum
Collateralized Debt Obligations [Abstract]
Interest rate								2.93%
8/2/2019 | Committed Loan Repurchase Facility | Maximum
Collateralized Debt Obligations [Abstract]
Interest rate								3.68%
10/24/2016 | Committed Loan Repurchase Facility
Committed Loan and Securities Repurchase Facilities
Committed amount or master repurchase facility terminated																	$ 35,000,000
4/10/2018 | Committed Loan Repurchase Facility
Committed Loan and Securities Repurchase Facilities
Committed amount on master repurchase agreement			$ 300,000,000.0				$ 300,000,000
Number of extension maturity periods | Extension							2
Number of additional extension maturity periods | Extension		1					1
Length of additional extension maturity periods		364 days					364 days
Gross amounts of recognized liabilities							$ 63,007,000
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	1 year
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%
10/30/2018 | Committed Loan Repurchase Facility
Committed Loan and Securities Repurchase Facilities
Committed amount on master repurchase agreement								$ 600,000,000
Debt borrowings term				1 year
Number of extension maturity periods | Extension								3
Length of extension options								12 months
Gross amounts of recognized liabilities								$ 183,604,000
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet								$ 292,628,000
10/30/2018 | Committed Loan Repurchase Facility | Minimum
Collateralized Debt Obligations [Abstract]
Interest rate								2.45%
10/30/2018 | Committed Loan Repurchase Facility | Maximum
Collateralized Debt Obligations [Abstract]
Interest rate								3.27%
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 amount on master repurchase agreement							$ 600,000,000
Number of extension maturity periods | Extension							2
Length of extension options							12 months
Length of additional extension maturity periods		12 months
Gross amounts of recognized liabilities							$ 120,493,000
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet							$ 160,031,000
10/1/2020 | Committed Loan Repurchase Facility | Minimum
Collateralized Debt Obligations [Abstract]
Interest rate							3.23%
10/1/2020 | Committed Loan Repurchase Facility | Maximum
Collateralized Debt Obligations [Abstract]
Interest rate							3.98%
Various Dates | Uncommitted securities Repurchase Facilities
Committed Loan and Securities Repurchase Facilities
Gross amounts of recognized liabilities							$ 74,757,000	$ 311,705,000
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet							$ 86,322,000	$ 368,638,000
Various Dates | Uncommitted securities Repurchase Facilities | Minimum
Collateralized Debt Obligations [Abstract]
Interest rate							1.65%	1.00%
Various Dates | Uncommitted securities Repurchase Facilities | Maximum
Collateralized Debt Obligations [Abstract]
Interest rate							3.31%	2.41%
Various Dates | CLO Debt
Committed Loan and Securities Repurchase Facilities
Committed amount on master repurchase agreement							$ 688,479,000
Gross amounts of recognized liabilities							688,479,000
Collateralized Debt Obligations [Abstract]
Unamortized debt issuance costs							6,000,000
Gross amounts offset in the balance sheet							$ 880,385,000						$ 431,500,000	$ 456,900,000
Various Dates | CLO Debt | Minimum
Collateralized Debt Obligations [Abstract]
Interest rate							2.36%
Various Dates | CLO Debt | Maximum
Collateralized Debt Obligations [Abstract]
Interest rate							5.08%
Various Dates | CLO Debt | Weighted Average
Collateralized Debt Obligations [Abstract]
Interest rate							3.40%
Various Dates | Mortgage loan financing
Committed Loan and Securities Repurchase Facilities
Committed amount on master repurchase agreement							$ 692,696,000	$ 590,106,000
Gross amounts of recognized liabilities							692,696,000	590,106,000
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet							$ 911,034,000	$ 757,468,000
Various Dates | Mortgage loan financing | Minimum
Collateralized Debt Obligations [Abstract]
Interest rate							4.25%	4.25%
Various Dates | Mortgage loan financing | Maximum
Collateralized Debt Obligations [Abstract]
Interest rate							6.75%	6.75%
6/6/2018 | Participation Financing - Mortgage Loan Receivable
Committed Loan and Securities Repurchase Facilities
Committed amount on master repurchase agreement							$ 3,107,000
Gross amounts of recognized liabilities							3,107,000
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet							$ 3,107,000
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Stated interest rate on debt instrument (as a percent)							17.00%
Credit Agreement and Revolving Credit Facility
Borrowing Under Credit Facilties and Debt Issuance Costs [Abstract]
Unamortized debt issuance costs							$ 7,800,000	$ 4,900,000
Mortgage loan financing
Committed Loan and Securities Repurchase Facilities
Number of agreements | agreement							28	18	51
Mortgage loan receivables held for investment, net, at amortized cost:
Mortgage loan financing							$ 692,700,000	$ 590,100,000
Net unamortized premiums							6,600,000	5,600,000
Amortization of premiums							1,000,000	900,000	$ 900,000
Pledged assets, real estate and lease intangibles, net							$ 911,000,000	$ 757,500,000
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)							6.75%
Borrowings from the Federal Home Loan Bank | Tuebor
Committed Loan and Securities Repurchase Facilities
Debt borrowings term							6 years	7 years
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Advance rates of total assets (as a percent)	40.00%
Advance rates of total equity (as a percent)	150.00%
FHLB borrowings outstanding							$ 1,400,000,000	$ 1,700,000,000
Additional committed term financing available from FHLB							$ 630,000,000	$ 338,900,000
Weighted average term							2 years 5 months 26 days	2 years 5 months 1 day
Weighted average interest rate							1.61%	1.12%
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)							49.60%	49.60%
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Stated interest rate on debt instrument (as a percent)							0.87%	0.43%
Borrowings from the Federal Home Loan Bank | Tuebor | Maximum
Borrowing Under Credit Facilties and Debt Issuance Costs [Abstract]
Advance rates (as a percent)							100.00%	95.20%
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Stated interest rate on debt instrument (as a percent)							2.74%	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							$ 861,700,000	$ 1,400,000,000
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							$ 915,900,000	$ 724,000,000
Affiliated Entity | Various Dates | 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	$ 2,000,000,000