DEBT OBLIGATIONS - Schedule of Company's Debt Obligations (Details)				9 Months Ended	12 Months Ended
	Aug. 03, 2016 USD ($) Extension	Apr. 19, 2016 Extension	Oct. 25, 2015 USD ($) Extension	Sep. 30, 2016 USD ($) Extension	Dec. 31, 2015 USD ($) Extension	Jul. 01, 2016 USD ($)	Jun. 28, 2016 USD ($)	Aug. 14, 2015 USD ($) Extension	Apr. 10, 2015 USD ($)	Feb. 19, 2015 Extension
Assets Sold under Agreements to Repurchase [Line Items]
Debt obligations outstanding				$ 1,458,327,000	$ 1,260,755,000
Debt obligations				4,601,011,000	4,274,723,000
Carrying Amount of Collateral				0	0
Unamortized debt issuance costs				(4,598,000)
Committed Loan Repurchase Facilities
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement				1,650,000,000	1,485,000,000
Debt obligations outstanding				622,643,000	704,149,000
Committed but Unfunded				1,027,357,000	780,851,000
Carrying Amount of Collateral				1,046,140,000	1,101,593,000
Fair Value of Collateral				1,097,651,000	1,176,417,000
Committed Loan Repurchase Facilities | 10/30/2017
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement				600,000,000
Debt obligations outstanding				276,978,000
Committed but Unfunded				323,022,000
Carrying Amount of Collateral				442,221,000
Fair Value of Collateral				$ 447,038,000
Number of extension maturity periods | Extension				1
Length of extension options				12 months
Committed Loan Repurchase Facilities | 10/30/2017 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				2.27%
Committed Loan Repurchase Facilities | 10/30/2017 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				3.03%
Committed Loan Repurchase Facilities | 5/24/2017
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement				$ 450,000,000
Debt obligations outstanding				187,031,000
Committed but Unfunded				262,969,000
Carrying Amount of Collateral				271,091,000
Fair Value of Collateral				$ 290,496,000
Number of extension maturity periods | Extension		2		3
Length of extension options		1 year		12 months
Committed Loan Repurchase Facilities | 5/24/2017 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				2.77%
Committed Loan Repurchase Facilities | 5/24/2017 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				3.52%
Committed Loan Repurchase Facilities | 5/24/2017 | Consolidated Subsidiaries
Assets Sold under Agreements to Repurchase [Line Items]
Debt obligations outstanding				$ 16,400,000
Committed Loan Repurchase Facilities | 4/9/2017
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement				400,000,000					$ 400,000,000
Debt obligations outstanding				120,690,000
Committed but Unfunded				279,310,000
Carrying Amount of Collateral				281,269,000
Fair Value of Collateral				$ 303,158,000
Number of extension maturity periods | Extension				2
Length of extension options				364 days
Committed Loan Repurchase Facilities | 4/9/2017 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				2.77%
Committed Loan Repurchase Facilities | 4/9/2017 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				3.77%
Committed Loan Repurchase Facilities | 4/9/2017 | Consolidated Subsidiaries
Assets Sold under Agreements to Repurchase [Line Items]
Debt obligations outstanding				$ 16,100,000
Committed Loan Repurchase Facilities | 6/28/2019
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement				100,000,000			$ 100,000,000
Debt obligations outstanding				0
Committed but Unfunded				100,000,000
Carrying Amount of Collateral				0
Fair Value of Collateral				0
Committed Loan Repurchase Facilities | 8/2/2019
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement	$ 100,000,000			100,000,000
Debt obligations outstanding				37,944,000
Committed but Unfunded				62,056,000
Carrying Amount of Collateral				51,559,000
Fair Value of Collateral				$ 56,959,000
Number of extension maturity periods | Extension	1			1
Length of extension options	12 months			12 months
Number of additional extension maturity periods | Extension	2			2
Length of additional extension maturity periods	6 months			6 months
Committed Loan Repurchase Facilities | 8/2/2019 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				2.81%
Committed Loan Repurchase Facilities | 8/2/2019 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				3.53%
Committed Loan Repurchase Facilities | 8/2/2019 | Consolidated Subsidiaries
Assets Sold under Agreements to Repurchase [Line Items]
Debt obligations outstanding				$ 5,200,000
Committed Loan Repurchase Facilities | 4/10/2016
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement					400,000,000
Debt obligations outstanding					204,262,000
Committed but Unfunded					195,738,000
Carrying Amount of Collateral					299,714,000
Fair Value of Collateral					$ 342,307,000
Number of extension maturity periods | Extension					2
Length of extension options					364 days
Committed Loan Repurchase Facilities | 4/10/2016 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)					2.44%
Committed Loan Repurchase Facilities | 4/10/2016 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)					4.33%
Committed Loan Repurchase Facilities | 4/10/2016 | Consolidated Subsidiaries
Assets Sold under Agreements to Repurchase [Line Items]
Debt obligations outstanding					$ 36,500,000
Committed Loan Repurchase Facilities | 5/24/2016
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement					450,000,000
Debt obligations outstanding					269,779,000
Committed but Unfunded					180,221,000
Carrying Amount of Collateral					436,901,000
Fair Value of Collateral					$ 466,640,000
Number of extension maturity periods | Extension					2
Length of extension options					12 months
Committed Loan Repurchase Facilities | 5/24/2016 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)					2.58%
Committed Loan Repurchase Facilities | 5/24/2016 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)					4.33%
Committed Loan Repurchase Facilities | 5/24/2016 | Consolidated Subsidiaries
Assets Sold under Agreements to Repurchase [Line Items]
Debt obligations outstanding					$ 28,200,000
Committed Loan Repurchase Facilities | 10/30/2016
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement					600,000,000			$ 600,000,000
Debt obligations outstanding					229,533,000
Committed but Unfunded					370,467,000
Carrying Amount of Collateral					364,978,000
Fair Value of Collateral					$ 366,676,000
Number of extension maturity periods | Extension					2			1
Length of extension options					12 months
Committed Loan Repurchase Facilities | 10/30/2016 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)					2.08%
Committed Loan Repurchase Facilities | 10/30/2016 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)					2.93%
Committed Loan Repurchase Facilities | 10/24/2016
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement			$ 35,000,000		$ 35,000,000
Debt obligations outstanding					575,000
Committed but Unfunded					$ 34,425,000
Interest Rate(s) (as a percent)					3.02%
Carrying Amount of Collateral					$ 0
Fair Value of Collateral					$ 794,000
Number of extension maturity periods | Extension			2		2
Length of extension options			6 months		6 months
Committed Loan Repurchase Facilities | 10/24/2016 | Consolidated Subsidiaries
Assets Sold under Agreements to Repurchase [Line Items]
Debt obligations outstanding					$ 800,000
Committed Securities Repurchase Facility
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement				400,000,000
Number of extension maturity periods | Extension										1
Committed Securities Repurchase Facility | 10/31/2016
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement					300,000,000
Debt obligations outstanding					161,887,000
Committed but Unfunded					138,113,000
Carrying Amount of Collateral					193,530,000
Fair Value of Collateral					$ 193,530,000
Committed Securities Repurchase Facility | 10/31/2016 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)					0.88%
Committed Securities Repurchase Facility | 10/31/2016 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)					1.34%
Committed Securities Repurchase Facility | 7/1/2018
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement				400,000,000		$ 400,000,000
Debt obligations outstanding				380,319,000
Committed but Unfunded				19,681,000
Carrying Amount of Collateral				442,288,000
Fair Value of Collateral				$ 442,288,000
Committed Securities Repurchase Facility | 7/1/2018 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				0.88%
Committed Securities Repurchase Facility | 7/1/2018 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				2.29%
Uncommitted Securities Repurchase Facilities | 1/2016
Assets Sold under Agreements to Repurchase [Line Items]
Debt obligations outstanding					$ 394,719,000
Carrying Amount of Collateral					458,615,000
Fair Value of Collateral					$ 458,615,000
Uncommitted Securities Repurchase Facilities | 1/2016 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)					0.73%
Uncommitted Securities Repurchase Facilities | 1/2016 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)					2.02%
Uncommitted Securities Repurchase Facilities | Various Dates
Assets Sold under Agreements to Repurchase [Line Items]
Debt obligations outstanding				$ 455,365,000
Carrying Amount of Collateral				534,336,000
Fair Value of Collateral				$ 534,336,000
Uncommitted Securities Repurchase Facilities | Various Dates | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				0.80%
Uncommitted Securities Repurchase Facilities | Various Dates | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				2.33%
Total Repurchase Facilities
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement				$ 2,050,000,000	$ 1,785,000,000
Debt obligations outstanding				1,458,327,000	1,260,755,000
Committed but Unfunded				1,047,038,000	918,964,000
Carrying Amount of Collateral				2,022,764,000	1,753,738,000
Fair Value of Collateral				2,074,275,000	1,828,562,000
Credit Agreement | 1/24/2016
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement					50,000,000
Debt obligations outstanding					0
Committed but Unfunded					50,000,000
Carrying Amount of Collateral					0
Fair Value of Collateral					0
Revolving Credit Facility | 2/11/2017
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement				143,000,000	75,000,000
Debt obligations outstanding				100,000,000	0
Committed but Unfunded				$ 43,000,000	$ 75,000,000
Number of extension maturity periods | Extension					2
Length of extension options					12 months
Revolving Credit Facility | 2/11/2017 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				3.02%
Revolving Credit Facility | 2/11/2017 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				3.69%
Mortgage Loan Financing | 2/11/2017
Assets Sold under Agreements to Repurchase [Line Items]
Number of extension maturity periods | Extension				1
Length of extension options				12 months
Mortgage Loan Financing | Various Dates
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement				$ 575,533,465	$ 544,662,887
Debt obligations outstanding				575,533,000	544,663,000
Committed but Unfunded				0	0
Carrying Amount of Collateral				746,082,000	711,090,000
Fair Value of Collateral				$ 851,061,000	$ 788,369,000
Mortgage Loan Financing | Various Dates | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				4.25%	4.25%
Mortgage Loan Financing | Various Dates | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				6.75%	6.75%
Mortgage Loan Receivable Financing
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement				$ 63,176,616
Debt obligations outstanding				63,177,000
Committed but Unfunded				0
Borrowings from the FHLB | Various Dates
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement				2,251,045,764	$ 2,237,113,311
Debt obligations outstanding				1,844,700,000	1,856,700,000
Committed but Unfunded				406,346,000	380,413,000
Carrying Amount of Collateral				2,354,675,000	2,317,534,000
Fair Value of Collateral				$ 2,371,672,000	$ 2,323,765,000
Borrowings from the FHLB | Various Dates | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				0.38%	0.28%
Borrowings from the FHLB | Various Dates | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)				3.02%	2.74%
Senior Unsecured Notes | Various Dates
Assets Sold under Agreements to Repurchase [Line Items]
Debt issued				$ 563,872,000
Senior unsecured notes				559,274,000
Committed but Unfunded				0	$ 0
Unamortized debt issuance costs				$ (4,600,000)	$ (6,900,000)
Senior Unsecured Notes | Various Dates | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Stated interest rate on debt instrument (as a percent)				5.875%	5.875%
Senior Unsecured Notes | Various Dates | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Stated interest rate on debt instrument (as a percent)				7.375%	7.375%
Senior Unsecured Notes | Various Dates | Senior unsecured notes
Assets Sold under Agreements to Repurchase [Line Items]
Debt issued					$ 619,555,000
Senior Unsecured Notes | Various Dates | Senior unsecured notes | Broker quotations, pricing services | Recurring
Assets Sold under Agreements to Repurchase [Line Items]
Senior unsecured notes					612,605,000
Total Debt Obligations
Assets Sold under Agreements to Repurchase [Line Items]
Debt issued				$ 5,646,628,000	5,311,331,000
Debt obligations				4,601,011,000	4,274,723,000
Committed but Unfunded				1,496,384,000	1,424,377,000
Carrying Amount of Collateral				5,123,521,000	4,782,362,000
Fair Value of Collateral				$ 5,297,008,000	4,940,696,000
Senior Unsecured Notes | Various Dates
Assets Sold under Agreements to Repurchase [Line Items]
Unamortized debt issuance costs					$ (6,900,000)