DEBT OBLIGATIONS, NET - Additional Information (Details)							3 Months Ended		9 Months Ended		12 Months Ended
	Mar. 21, 2017 Extension	Feb. 22, 2017 USD ($)	Jan. 13, 2017 USD ($)	Nov. 09, 2016	Apr. 19, 2016 Extension	Feb. 11, 2014 USD ($) Extension	Sep. 30, 2017 USD ($) counterparty Extension	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) counterparty agreement Extension	Sep. 30, 2016 USD ($) agreement	Dec. 31, 2016 USD ($) Extension	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							$ 913,137,000		$ 913,137,000		$ 1,107,185,000
Mortgage loan receivables held for investment, net, at amortized cost:
Amortization of premiums									(681,000)	$ (660,000)
Participation Financing - Mortgage Loan Receivable
Proceeds from borrowings under debt obligations									$ 8,248,079,000	$ 9,290,374,000
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Percent of FHLB advances to total debt obligations outstanding							30.30%		30.30%
Retained earnings, appropriated							$ 780,000,000.0		$ 780,000,000.0
Tuebor
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Amount restricted from transfer							336,200,000		$ 336,200,000
Revolving credit facility | One-Month LIBOR
Committed Loan and Securities Repurchase Facilities
Committed amount on credit agreement						$ 215,500,000
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						$ 25,000,000.0
Committed Loan Repurchase Facility
Committed Loan and Securities Repurchase Facilities
Number of agreements | agreement									5
Committed amount on credit agreement							1,700,000,000		$ 1,700,000,000
Committed amount on master repurchase agreement							1,650,000,000		1,650,000,000		1,650,000,000
Gross amounts of recognized liabilities							696,394,000		696,394,000		567,163,000
Term master repurchase agreement
Committed Loan and Securities Repurchase Facilities
Committed amount on master repurchase agreement							400,000,000.0		400,000,000.0
Total Repurchase Facilities
Committed Loan and Securities Repurchase Facilities
Committed amount on master repurchase agreement							$ 2,050,000,000		$ 2,050,000,000		2,050,000,000
Repurchase agreements, number of counterparties | counterparty							8		8
Gross amounts of recognized liabilities							$ 913,137,000		$ 913,137,000		1,107,185,000
Total Repurchase Facilities | Deutshe Bank, J.P. Morgan and Wells Fargo
Committed Loan and Securities Repurchase Facilities
Repurchase agreements, number of counterparties | counterparty							3		3
Excess collateral over amounts borrowed under repurchase agreements							$ 73,300,000		$ 73,300,000
Ratio indebtedness over total equity (as a percent)									5.00%
Haircut on repurchase agreements (as a percent)							34.30%		34.30%
Uncommitted securities Repurchase Facilities | Minimum
Borrowing Under Credit Facilties and Debt Issuance Costs [Abstract]
Advance rates (as a percent)									70.00%
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									$ 4,000,000.0
Interest Expense, Related Party							$ 200,000
Interest expense									400,000
5/24/2018 | Committed Loan Repurchase Facility
Committed Loan and Securities Repurchase Facilities
Committed amount on master repurchase agreement							$ 450,000,000		$ 450,000,000
Debt borrowings term					1 year
Number of extension maturity periods | Extension					2		3		3
Length of extension options									12 months
Gross amounts of recognized liabilities							$ 179,645,000		$ 179,645,000
9/30/2018 | Term master repurchase agreement
Committed Loan and Securities Repurchase Facilities
Committed amount on master repurchase agreement											400,000,000				$ 400,000,000.0
Gross amounts of recognized liabilities											228,317,000
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					$ 100,000,000.0
Gross amounts of recognized liabilities							31,370,000		31,370,000		27,132,000
8/2/2019 | Committed Loan Repurchase Facility
Committed Loan and Securities Repurchase Facilities
Committed amount on master repurchase agreement											$ 100,000,000			$ 100,000,000.0
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
10/24/2016 | Committed Loan Repurchase Facility
Committed Loan and Securities Repurchase Facilities
Committed amount or master repurchase facility terminated														$ 35,000,000.0
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
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		$ 300,000,000
Number of extension maturity periods | Extension							2		2
Number of additional extension maturity periods | Extension	1						1		1
Length of additional extension maturity periods	364 days								364 days
Gross amounts of recognized liabilities							$ 132,441,000		$ 132,441,000
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
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		$ 600,000,000
Number of extension maturity periods | Extension							2		2
Length of extension options									12 months
Length of additional extension maturity periods	12 months
Gross amounts of recognized liabilities							$ 283,389,000		$ 283,389,000
Credit Agreement and Revolving Credit Facility
Borrowing Under Credit Facilties and Debt Issuance Costs [Abstract]
Unamortized debt issuance costs							5,100,000		$ 5,100,000		4,900,000
Mortgage loan financing
Committed Loan and Securities Repurchase Facilities
Number of agreements | agreement									0	10
Mortgage loan receivables held for investment, net, at amortized cost:
Mortgage loan financing							587,500,000		$ 587,500,000		590,100,000
Net unamortized premiums							4,900,000		4,900,000		5,600,000
Amortization of premiums							200,000	$ 200,000	700,000	$ 700,000
Pledged assets, real estate and lease intangibles, net							$ 736,100,000		$ 736,100,000		$ 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%		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%		6.75%
Borrowings from the Federal Home Loan Bank | Tuebor
Committed Loan and Securities Repurchase Facilities
Debt borrowings term									7 years		7 years
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Maximum advance limit			$ 2,000,000,000.0
Advance rates of total assets (as a percent)			40.00%
Advance rates of total equity (as a percent)			150.00%
FHLB borrowings outstanding							$ 1,500,000,000		$ 1,500,000,000		$ 1,700,000,000
Additional committed term financing available from FHLB							$ 536,000,000.0		$ 536,000,000.0		$ 338,900,000
Weighted average term									2 years 6 months		2 years 4 months 24 days
Weighted average interest rate							1.50%		1.50%		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)									57.80%		49.60%
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Stated interest rate on debt instrument (as a percent)							0.87%		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)									95.20%		95.20%
Borrowings from Federal Home Loan Bank (FHLB) [Abstract]
Stated interest rate on debt instrument (as a percent)							2.74%		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							$ 866,600,000		$ 866,600,000		$ 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							$ 1,100,000,000		$ 1,100,000,000		$ 724,000,000.0