DEBT OBLIGATIONS - Schedule of Company's Debt Obligations (Details)		6 Months Ended	12 Months Ended
	Oct. 25, 2015 USD ($) Extension	Jun. 30, 2016 USD ($) Extension	Dec. 31, 2015 USD ($) Extension	Jun. 28, 2016 USD ($)	Aug. 14, 2015 USD ($)	Apr. 10, 2015 USD ($)
Assets Sold under Agreements to Repurchase [Line Items]
Debt obligations outstanding		$ 1,139,615,000	$ 1,260,755,000
Debt obligations		4,394,969,000	4,274,723,000
Carrying Amount of Collateral		0	0
Unamortized debt issuance costs		(5,172,000)
Committed Loan Repurchase Facilities
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		1,585,000,000	1,485,000,000
Debt obligations outstanding		513,229,000	704,149,000
Committed but Unfunded		1,071,771,000	780,851,000
Carrying Amount of Collateral		912,021,000	1,101,593,000
Fair Value of Collateral		946,595,000	1,176,417,000
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		$ 600,000,000
Debt obligations outstanding		282,099,000	229,533,000
Committed but Unfunded		317,901,000	370,467,000
Carrying Amount of Collateral		444,308,000	364,978,000
Fair Value of Collateral		$ 448,105,000	$ 366,676,000
Number of extension maturity periods | Extension		2	2
Length of extension options		12 months	12 months
Committed Loan Repurchase Facilities | 10/30/2016 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		2.19%	2.08%
Committed Loan Repurchase Facilities | 10/30/2016 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		2.95%	2.93%
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		116,043,000
Committed but Unfunded		333,957,000
Carrying Amount of Collateral		196,719,000
Fair Value of Collateral		$ 196,892,000
Number of extension maturity periods | Extension		3	2
Length of extension options		12 months	12 months
Committed Loan Repurchase Facilities | 5/24/2017 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		2.70%
Committed Loan Repurchase Facilities | 5/24/2017 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		3.45%
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		110,735,000
Committed but Unfunded		289,265,000
Carrying Amount of Collateral		270,994,000
Fair Value of Collateral		$ 295,110,000
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)		2.69%
Committed Loan Repurchase Facilities | 4/9/2017 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		3.69%
Committed Loan Repurchase Facilities | 4/9/2017 | Consolidated Subsidiaries
Assets Sold under Agreements to Repurchase [Line Items]
Debt obligations outstanding		$ 23,400,000
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 | 6/28/2019
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		100,000,000		$ 100,000,000.0
Debt obligations outstanding		0
Committed but Unfunded		100,000,000
Carrying Amount of Collateral		0
Fair Value of Collateral		0
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	35,000,000
Debt obligations outstanding		4,352,000	575,000
Committed but Unfunded		$ 30,648,000	$ 34,425,000
Interest Rate(s) (as a percent)		3.05%	3.02%
Carrying Amount of Collateral		$ 0	$ 0
Fair Value of Collateral		$ 6,488,000	$ 794,000
Number of extension maturity periods | Extension	2	2	2
Length of extension options	6 months	6 months	6 months
Committed Loan Repurchase Facilities | 10/24/2016 | Consolidated Subsidiaries
Assets Sold under Agreements to Repurchase [Line Items]
Debt obligations outstanding		$ 6,500,000	$ 800,000
Committed Loan Repurchase Facilities | 10/31/2016 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)			0.88%
Committed Loan Repurchase Facilities | 10/31/2016 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)			1.34%
Committed Securities Repurchase Facility
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		300,000,000
Committed Securities Repurchase Facility | 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		247,274,000	161,887,000
Committed but Unfunded		52,726,000	138,113,000
Carrying Amount of Collateral		293,422,000	193,530,000
Fair Value of Collateral		$ 293,422,000	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)		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		$ 379,112,000
Carrying Amount of Collateral		452,230,000
Fair Value of Collateral		$ 452,230,000
Uncommitted Securities Repurchase Facilities | Various Dates | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		0.70%
Uncommitted Securities Repurchase Facilities | Various Dates | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		2.17%
Total Repurchase Facilities
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		$ 1,885,000,000	$ 1,785,000,000
Debt obligations outstanding		1,139,615,000	1,260,755,000
Committed but Unfunded		1,124,497,000	918,964,000
Carrying Amount of Collateral		1,657,673,000	1,753,738,000
Fair Value of Collateral		1,692,247,000	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
Interest Rate(s) (as a percent)		3.69%
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		$ 546,953,446	544,662,887
Debt obligations outstanding		546,953,000	544,663,000
Committed but Unfunded		0	0
Carrying Amount of Collateral		711,385,000	711,090,000
Fair Value of Collateral		$ 806,889,000	$ 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		$ 2,229,148,427	$ 2,237,113,311
Debt obligations outstanding		2,049,701,000	1,856,700,000
Committed but Unfunded		179,447,000	380,413,000
Carrying Amount of Collateral		2,550,739,000	2,317,534,000
Fair Value of Collateral		$ 2,569,830,000	$ 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 unsecured notes		558,700,000
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 unsecured notes			612,605,000
Total Debt Obligations
Assets Sold under Agreements to Repurchase [Line Items]
Debt issued		$ 5,367,974,000	5,311,331,000
Debt obligations		4,394,969,000	4,274,723,000
Committed but Unfunded		1,346,944,000	1,424,377,000
Carrying Amount of Collateral		4,919,797,000	4,782,362,000
Fair Value of Collateral		5,068,966,000	4,940,696,000
Senior Unsecured Notes | Various Dates
Assets Sold under Agreements to Repurchase [Line Items]
Unamortized debt issuance costs		$ (5,200,000)	$ (6,900,000)