DEBT OBLIGATIONS - Schedule of Uncommitted Securities Repurchase Facilities (Details)		3 Months Ended	12 Months Ended
	Oct. 25, 2015 USD ($) Extension	Mar. 31, 2016 USD ($) Extension	Dec. 31, 2015 USD ($) Extension	Aug. 14, 2015 USD ($)	Apr. 10, 2015 USD ($)
Assets Sold under Agreements to Repurchase [Line Items]
Debt obligations outstanding		$ 1,104,339,000	$ 1,260,755,000
Debt obligations		4,091,449,000	4,274,723,000
Carrying Amount of Collateral		0	0
Deferred finance costs, Net		(5,738,000)
Other Assets | Adjustments for New Accounting Principle, Early Adoption
Assets Sold under Agreements to Repurchase [Line Items]
Deferred finance costs, Net		5,700,000	6,900,000
Committed loan repurchase facility
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		1,485,000,000	1,485,000,000
Debt obligations outstanding		535,588,000	704,149,000
Committed but Unfunded		949,412,000	780,851,000
Carrying Amount of Collateral		946,251,000	1,101,593,000
Fair Value of Collateral		1,010,305,000	1,176,417,000
Committed loan repurchase facility | 10/30/2016
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		600,000,000	600,000,000	$ 600,000,000
Debt obligations outstanding		207,849,000	229,533,000
Committed but Unfunded		392,151,000	370,467,000
Carrying Amount of Collateral		339,106,000	364,978,000
Fair Value of Collateral		$ 340,373,000	$ 366,676,000
Number of extension maturity periods | Extension		2	2
Length of extension options		12 months	12 months
Committed loan repurchase facility | 10/30/2016 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		2.19%	2.08%
Committed loan repurchase facility | 10/30/2016 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		2.94%	2.93%
Committed loan repurchase facility | 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 facility | 4/10/2016 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)			2.44%
Committed loan repurchase facility | 4/10/2016 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)			4.33%
Committed loan repurchase facility | 4/10/2016 | Consolidated Subsidiaries
Assets Sold under Agreements to Repurchase [Line Items]
Debt obligations outstanding			$ 36,500,000
Committed loan repurchase facility | 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		176,746,000
Committed but Unfunded		223,254,000
Carrying Amount of Collateral		334,212,000
Fair Value of Collateral		$ 369,623,000
Number of extension maturity periods | Extension		2
Length of extension options		364 days
Committed loan repurchase facility | 4/9/2017 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		2.68%
Committed loan repurchase facility | 4/9/2017 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		4.44%
Committed loan repurchase facility | 4/9/2017 | Consolidated Subsidiaries
Assets Sold under Agreements to Repurchase [Line Items]
Debt obligations outstanding		$ 32,200,000
Committed loan repurchase facility | 5/24/2016
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		450,000,000	450,000,000
Debt obligations outstanding		148,731,000	269,779,000
Committed but Unfunded		301,269,000	180,221,000
Carrying Amount of Collateral		272,933,000	436,901,000
Fair Value of Collateral		$ 297,116,000	$ 466,640,000
Number of extension maturity periods | Extension		2
Length of extension options		12 months
Committed loan repurchase facility | 5/24/2016 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		2.69%	2.58%
Committed loan repurchase facility | 5/24/2016 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		4.36%	4.33%
Committed loan repurchase facility | 5/24/2016 | Consolidated Subsidiaries
Assets Sold under Agreements to Repurchase [Line Items]
Debt obligations outstanding		$ 22,900,000	$ 28,200,000
Committed loan repurchase facility | 10/24/2016
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement	$ 35,000,000	35,000,000	35,000,000
Debt obligations outstanding		2,262,000	575,000
Committed but Unfunded		32,738,000	$ 34,425,000
Interest Rate(s) (as a percent)			3.02%
Carrying Amount of Collateral		0	$ 0
Fair Value of Collateral		$ 3,193,000	$ 794,000
Number of extension maturity periods | Extension	2	2	2
Length of extension options	6 months	6 months	6 months
Committed loan repurchase facility | 10/24/2016 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		3.03%
Committed loan repurchase facility | 10/24/2016 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		3.04%
Committed loan repurchase facility | 10/24/2016 | Consolidated Subsidiaries
Assets Sold under Agreements to Repurchase [Line Items]
Debt obligations outstanding		$ 3,200,000	$ 800,000
Committed loan repurchase facility | 10/31/2016 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		0.88%	0.88%
Committed loan repurchase facility | 10/31/2016 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		2.43%	1.34%
Term master repurchase agreement
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		$ 300,000,000
Term master repurchase agreement | 10/31/2016
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		300,000,000	$ 300,000,000
Debt obligations outstanding		203,059,000	161,887,000
Committed but Unfunded		96,941,000	138,113,000
Carrying Amount of Collateral		245,022,000	193,530,000
Fair Value of Collateral		245,022,000	193,530,000
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 | 6/2016
Assets Sold under Agreements to Repurchase [Line Items]
Debt obligations outstanding		365,692,000
Carrying Amount of Collateral		435,941,000
Fair Value of Collateral		$ 435,941,000
Uncommitted securities repurchase facilities | 6/2016 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		0.75%
Uncommitted securities repurchase facilities | 6/2016 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		2.14%
Master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		$ 1,785,000,000	$ 1,785,000,000
Debt obligations outstanding		1,104,339,000	1,260,755,000
Committed but Unfunded		1,046,353,000	918,964,000
Carrying Amount of Collateral		1,627,214,000	1,753,738,000
Fair Value of Collateral		1,691,268,000	1,828,562,000
Borrowings under credit agreement | 4/24/2016
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
Borrowings under 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		0	0
Committed but Unfunded		$ 143,000,000	75,000,000
Number of extension maturity periods | Extension		2
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		$ 547,776,379	544,662,887
Debt obligations outstanding		547,776,000	544,663,000
Committed but Unfunded		0	0
Carrying Amount of Collateral		712,468,000	711,090,000
Fair Value of Collateral		$ 799,169,000	$ 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%
Borrowings from the FHLB | Various Dates
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		$ 2,221,954,722	$ 2,237,113,311
Debt obligations outstanding		1,881,200,000	1,856,700,000
Committed but Unfunded		340,755,000	380,413,000
Carrying Amount of Collateral		2,355,506,000	2,317,534,000
Fair Value of Collateral		$ 2,363,781,000	$ 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)		2.74%	2.74%
Senior Unsecured Notes | Various Dates
Assets Sold under Agreements to Repurchase [Line Items]
Debt issued		$ 563,872,000
Senior Notes		558,134,000
Committed but Unfunded		$ 0	$ 0
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 Notes			612,605,000
Total Debt Obligations
Assets Sold under Agreements to Repurchase [Line Items]
Debt issued		$ 5,311,603,000	5,311,331,000
Debt obligations		4,091,449,000	4,274,723,000
Committed but Unfunded		1,580,108,000	1,424,377,000
Carrying Amount of Collateral		4,695,188,000	4,782,362,000
Fair Value of Collateral		$ 4,854,218,000	$ 4,940,696,000