DEBT OBLIGATIONS, NET - Additional Information (Details)					3 Months Ended		12 Months Ended
	Dec. 06, 2017	Mar. 23, 2017 Extension	Mar. 21, 2017 Extension	Feb. 11, 2014 USD ($) Extension	Mar. 31, 2018 USD ($) counterparty agreement Extension	Mar. 31, 2017 USD ($) agreement	Dec. 31, 2017 USD ($) Extension	Jan. 01, 2021 USD ($)	Apr. 01, 2020 USD ($)	Dec. 31, 2018 USD ($)	Dec. 21, 2017 USD ($)	Oct. 17, 2017 USD ($)	Mar. 01, 2017 USD ($)	Feb. 22, 2017 USD ($)	Feb. 19, 2016
Committed Loan and Securities Repurchase Facilities
Committed amount on credit agreement					$ 450,000,000
Gross amounts of recognized liabilities					754,377,000		$ 473,410,000
Mortgage loan receivables held for investment, net, at amortized cost:
Amortization of premiums					(253,000)	$ (226,000)
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet					0		0
Proceeds from borrowings under debt obligations					$ 1,312,451,000	$ 2,666,161,000
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Percent of FHLB advances to total debt obligations outstanding					29.10%
Retained earnings, appropriated					$ 780,000,000
Tuebor
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Amount restricted from transfer					$ 1,300,000,000
Revolving credit facility | One-Month LIBOR
Committed Loan and Securities Repurchase Facilities
Committed amount on credit agreement				$ 241,430,000
Number of additional extension maturity periods | Extension		2
Period of additional extensions of maturity		1 year
Number of extension maturity periods | Extension				2
Length of extension options				12 months
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 Financing					1,650,000,000		1,650,000,000
Gross amounts of recognized liabilities					550,523,000		398,653,000
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet					914,280,000		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					$ 2,050,000,000		2,050,000,000
Repurchase agreements, number of counterparties | counterparty					8
Gross amounts of recognized liabilities					$ 754,377,000		473,410,000
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet					$ 1,153,849,000		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					$ 75,200,000
Ratio indebtedness over total equity (as a percent)					5.00%
Haircut on repurchase agreements (as a percent)					34.60%
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							2,800,000
Collateralized Debt Obligations [Abstract]
Proceeds from borrowings under debt obligations					$ 4,000,000
Interest expense, debt					$ 100,000
4/10/2018 | Committed Loan Repurchase Facility
Committed Loan and Securities Repurchase Facilities
Committed Financing							300,000,000							$ 300,000,000.0
Number of additional extension maturity periods | Extension			1
Length of additional extension maturity periods			364 days
Gross amounts of recognized liabilities							$ 63,007,000
Number of extension maturity periods | Extension							2
Length of extension options							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					3.78%		3.73%
4/10/2018 | Committed Loan Repurchase Facility | Maximum
Collateralized Debt Obligations [Abstract]
Interest rate					5.03%		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
Length of additional extension maturity periods			12 months
Gross amounts of recognized liabilities					$ 212,023,000		$ 120,493,000
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					$ 348,473,000		$ 160,031,000
10/1/2020 | Committed Loan Repurchase Facility | Minimum
Collateralized Debt Obligations [Abstract]
Interest rate					3.53%		3.23%
10/1/2020 | Committed Loan Repurchase Facility | Maximum
Collateralized Debt Obligations [Abstract]
Interest rate					4.28%		3.98%
Various Dates | Uncommitted securities Repurchase Facilities
Committed Loan and Securities Repurchase Facilities
Gross amounts of recognized liabilities					$ 105,092,000		$ 74,757,000
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet					$ 119,969,000		$ 86,322,000
Various Dates | Uncommitted securities Repurchase Facilities | Minimum
Collateralized Debt Obligations [Abstract]
Interest rate					2.39%		1.65%
Various Dates | Uncommitted securities Repurchase Facilities | Maximum
Collateralized Debt Obligations [Abstract]
Interest rate					3.69%		3.31%
Various Dates | Mortgage loan financing
Committed Loan and Securities Repurchase Facilities
Committed Financing					$ 683,487,000		$ 692,696,000
Gross amounts of recognized liabilities					683,487,000		692,696,000
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet					$ 895,786,000		$ 911,034,000
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%
Various Dates | CLO Debt
Committed Loan and Securities Repurchase Facilities
Committed Financing					$ 683,095,000		$ 688,479,000
Gross amounts of recognized liabilities					683,095,000		688,479,000
Collateralized Debt Obligations [Abstract]
Unamortized debt issuance costs					5,400,000		6,000,000
Gross amounts offset in the balance sheet					$ 881,719,000		$ 880,385,000				$ 431,500,000	$ 456,900,000
Various Dates | CLO Debt | Minimum
Collateralized Debt Obligations [Abstract]
Interest rate					2.67%		2.36%
Various Dates | CLO Debt | Maximum
Collateralized Debt Obligations [Abstract]
Interest rate					5.39%		5.08%
Various Dates | CLO Debt | Weighted Average
Collateralized Debt Obligations [Abstract]
Interest rate					3.64%
6/6/2018 | Participation Financing - Mortgage Loan Receivable
Committed Loan and Securities Repurchase Facilities
Committed Financing					$ 2,815,000		$ 3,107,000
Gross amounts of recognized liabilities					2,815,000		3,107,000
Collateralized Debt Obligations [Abstract]
Gross amounts offset in the balance sheet					$ 2,815,000		$ 3,107,000
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Stated interest rate on debt instrument (as a percent)					17.00%		17.00%
Credit Agreement and Revolving Credit Facility
Borrowing Under Credit Facilties and Debt Issuance Costs [Abstract]
Unamortized debt issuance costs					$ 7,400,000		$ 7,800,000
Mortgage loan financing
Committed Loan and Securities Repurchase Facilities
Number of agreements | agreement					3	0
Mortgage loan receivables held for investment, net, at amortized cost:
Mortgage loan financing					$ 683,500,000		692,700,000
Net unamortized premiums					6,300,000		6,600,000
Amortization of premiums					300,000	$ 200,000
Pledged assets, real estate and lease intangibles, net					$ 895,800,000		$ 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)							6.75%
Borrowings from the Federal Home Loan Bank | Tuebor
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					$ 1,300,000,000		$ 1,400,000,000
Additional committed term financing available from FHLB					$ 585,500,000		$ 630,000,000
Debt borrowings term					6 years	6 years
Weighted average term					2 years 5 months 8 days	2 years 5 months 26 days
Weighted average interest rate					1.82%		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)					52.00%	49.60%
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Stated interest rate on debt instrument (as a percent)					0.87%		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)					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					$ 837,200,000		$ 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					$ 990,100,000		$ 915,900,000
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