DEBT OBLIGATIONS - Schedule of Uncommitted Securities Repurchase Facilities (Details)		12 Months Ended
	Oct. 25, 2015 USD ($) Extension	Dec. 31, 2015 USD ($) period	Dec. 31, 2014 USD ($) period	Oct. 30, 2014 USD ($)	Apr. 29, 2014 USD ($)	Apr. 28, 2014 USD ($)
Assets Sold under Agreements to Repurchase [Line Items]
Debt obligations outstanding		$ 1,260,755,000	$ 1,431,665,000
Debt obligations		4,274,723,000	4,182,954,000
Carrying Amount of Collateral		0	0
Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		1,485,000,000	1,150,000,000	$ 450,000,000	$ 450,000,000	$ 300,000,000
Debt obligations outstanding		704,149,000	509,023,000
Committed but Unfunded		780,851,000	640,977,000
Carrying Amount of Collateral		1,101,593,000	801,961,000
Fair Value of Collateral		1,176,417,000	806,014,000
Term master repurchase agreement
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		300,000,000
Master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		1,785,000,000	1,450,000,000
Debt obligations outstanding		1,260,755,000	1,431,665,000
Committed but Unfunded		918,964,000	766,124,000
Carrying Amount of Collateral		1,753,738,000	1,878,034,000
Fair Value of Collateral		1,828,562,000	1,882,087,000
Total Debt Obligations
Assets Sold under Agreements to Repurchase [Line Items]
Debt issued		5,311,331,000	4,588,715,000
Debt obligations		4,274,723,000	4,182,954,000
Committed but Unfunded		1,424,377,000	1,144,124,000
Carrying Amount of Collateral		4,782,362,000	4,593,410,000
Fair Value of Collateral		4,940,696,000	4,648,313,000
10/30/2016 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		600,000,000	450,000,000
Debt obligations outstanding		229,533,000	147,796,000
Committed but Unfunded		370,467,000	302,204,000
Carrying Amount of Collateral		364,978,000	278,530,000
Fair Value of Collateral		$ 366,676,000	$ 279,921,000
Remaining Extension Options | period		2	2
Length of extension options		12 months	12 months
4/10/2016 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		$ 400,000,000	$ 250,000,000
Debt obligations outstanding		204,262,000	138,711,000
Committed but Unfunded		195,738,000	111,289,000
Carrying Amount of Collateral		299,714,000	144,858,000
Fair Value of Collateral		$ 342,307,000	$ 145,749,000
Remaining Extension Options | period		3	1
Length of extension options		364 days	364 days
5/24/2016 | Committed master repurchase agreements
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
Remaining Extension Options | period		2
Length of extension options		12 months
10/24/2016 | Committed master repurchase agreements
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
Length of extension options	6 months
10/31/2016 | Term master repurchase agreement
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
1/2016 | Uncommitted securities repurchase facilities
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
1/24/2016 | Borrowings under credit agreement
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		50,000,000	$ 50,000,000
Debt obligations outstanding		0	11,000,000
Committed but Unfunded		50,000,000	$ 39,000,000
Interest Rate(s) (as a percent)			2.91%
Carrying Amount of Collateral		0	$ 0
Fair Value of Collateral		0	0
5/26/2015 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement			450,000,000
Debt obligations outstanding			222,516,000
Committed but Unfunded			227,484,000
Carrying Amount of Collateral			378,573,000
Fair Value of Collateral			$ 380,344,000
Remaining Extension Options | period			2
Length of extension options			12 months
2/11/2017 | Revolving credit facility
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		75,000,000	$ 75,000,000
Debt obligations outstanding		0	25,000,000
Committed but Unfunded		$ 75,000,000	$ 50,000,000
Remaining Extension Options | period		2	2
Length of extension options		12 months	12 months
4/30/2015 | Term master repurchase agreement
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement			$ 300,000,000
Debt obligations outstanding			174,853,000
Committed but Unfunded			125,147,000
Carrying Amount of Collateral			214,617,000
Fair Value of Collateral			214,617,000
10/6/2015 | Credit and Security Agreement
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement			46,750,000
Debt obligations outstanding			46,750,000
Committed but Unfunded			$ 0
Interest Rate(s) (as a percent)			2.01%
Carrying Amount of Collateral			$ 54,775,000
Fair Value of Collateral			$ 55,000,000
Remaining Extension Options | period			1
Length of extension options			12 months
Various Dates | Uncommitted securities repurchase facilities
Assets Sold under Agreements to Repurchase [Line Items]
Debt obligations outstanding			$ 747,789,000
Carrying Amount of Collateral			861,456,000
Fair Value of Collateral			861,456,000
Various Dates | Mortgage loan financing
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		$ 544,662,887	447,409,690
Debt obligations outstanding		544,663,000	447,410,000
Committed but Unfunded		0	0
Carrying Amount of Collateral		711,090,000	591,613,000
Fair Value of Collateral		788,369,000	637,271,000
Various Dates | Borrowings from the FHLB
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		2,237,113,311	1,900,000,000
Debt obligations outstanding		1,856,700,000	1,611,000,000
Committed but Unfunded		380,413,000	289,000,000
Carrying Amount of Collateral		2,317,534,000	2,068,988,000
Fair Value of Collateral		2,323,765,000	2,073,955,000
Various Dates | Senior Unsecured Notes
Assets Sold under Agreements to Repurchase [Line Items]
Debt issued		619,555,000
Debt obligations outstanding		$ 612,605,000	$ 610,129,000
Minimum | 10/30/2016 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		2.08%	2.42%
Minimum | 4/10/2016 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		2.44%	2.41%
Minimum | 5/24/2016 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		2.58%
Minimum | 10/31/2016 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		0.88%
Minimum | 1/2016 | Uncommitted securities repurchase facilities
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		0.73%
Minimum | 5/26/2015 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)			2.42%
Minimum | 2/11/2017 | Revolving credit facility
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)			3.66%
Minimum | 4/30/2015 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)			0.87%
Minimum | Various Dates | Uncommitted securities repurchase facilities
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)			0.50%
Minimum | Various Dates | Mortgage loan financing
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		4.25%	4.25%
Minimum | Various Dates | Borrowings from the FHLB
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		0.28%	0.30%
Minimum | Various Dates | Senior Unsecured Notes
Assets Sold under Agreements to Repurchase [Line Items]
Stated interest rate on debt instrument (as a percent)		5.875%	5.875%
Maximum | 10/30/2016 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		2.93%	2.66%
Maximum | 4/10/2016 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		4.33%	3.04%
Maximum | 5/24/2016 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		4.33%
Maximum | 10/31/2016 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		1.34%
Maximum | 1/2016 | Uncommitted securities repurchase facilities
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		2.02%
Maximum | 5/26/2015 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)			3.16%
Maximum | 2/11/2017 | Revolving credit facility
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)			5.75%
Maximum | 4/30/2015 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)			1.27%
Maximum | Various Dates | Uncommitted securities repurchase facilities
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)			1.66%
Maximum | Various Dates | Mortgage loan financing
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		6.75%	6.75%
Maximum | Various Dates | Borrowings from the FHLB
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		2.74%	2.74%
Maximum | Various Dates | Senior Unsecured Notes
Assets Sold under Agreements to Repurchase [Line Items]
Stated interest rate on debt instrument (as a percent)		7.375%	7.375%
Consolidated Subsidiaries | 4/10/2016 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Debt obligations outstanding		$ 36,500,000
Consolidated Subsidiaries | 5/24/2016 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Debt obligations outstanding		28,200,000
Consolidated Subsidiaries | 10/24/2016 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Debt obligations outstanding		800,000
Other Assets | Adjustments for New Accounting Principle, Early Adoption
Assets Sold under Agreements to Repurchase [Line Items]
Deferred Finance Costs, Net		$ 6,900,000	$ 9,400,000
Senior unsecured notes | Various Dates | Senior Unsecured Notes
Assets Sold under Agreements to Repurchase [Line Items]
Debt issued			619,555,000
Senior unsecured notes | Broker quotations, pricing services | Recurring | Various Dates | Senior Unsecured Notes
Assets Sold under Agreements to Repurchase [Line Items]
Debt obligations outstanding			$ 610,129,000