DEBT OBLIGATIONS - Schedule of Uncommitted Securities Repurchase Facilities (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 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]
Outstanding Amount	$ 1,191,326,000	$ 1,431,666,000
Carrying Amount of Collateral	0	0
Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement	1,450,000,000	1,150,000,000	$ 450,000,000	$ 450,000,000	$ 300,000,000
Outstanding Amount	624,011,000	509,023,000
Committed but Unfunded	825,989,000	640,977,000
Carrying Amount of Collateral	1,075,554,000	801,961,000
Fair Value of Collateral	1,083,081,000	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	1,750,000,000	1,450,000,000
Outstanding Amount	1,191,326,000	1,431,665,000
Committed but Unfunded	964,460,000	766,124,000
Carrying Amount of Collateral	1,748,495,000	1,878,034,000
Fair Value of Collateral	1,756,022,000	1,882,087,000
Total Debt Obligations
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement	4,679,498,000	3,969,160,000
Outstanding Amount	3,603,512,000	3,572,825,000
Committed but Unfunded	1,481,772,000	1,144,124,000
Carrying Amount of Collateral	4,744,214,000	4,593,410,000
Fair Value of Collateral	4,834,476,000	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
Outstanding Amount	218,516,000	147,796,000
Committed but Unfunded	381,484,000	302,204,000
Carrying Amount of Collateral	323,137,000	278,530,000
Fair Value of Collateral	$ 325,108,000	$ 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
Outstanding Amount	128,955,000	138,711,000
Committed but Unfunded	271,045,000	111,289,000
Carrying Amount of Collateral	253,930,000	144,858,000
Fair Value of Collateral	$ 255,235,000	$ 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
Outstanding Amount	276,540,000
Committed but Unfunded	173,460,000
Carrying Amount of Collateral	498,487,000
Fair Value of Collateral	$ 502,738,000
Remaining Extension Options | period	2
Length of extension options	12 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
Outstanding Amount	161,529,000
Committed but Unfunded	138,471,000
Carrying Amount of Collateral	193,271,000
Fair Value of Collateral	193,271,000
10/2015 | Uncommitted securities repurchase facilities
Assets Sold under Agreements to Repurchase [Line Items]
Outstanding Amount	405,786,000
Carrying Amount of Collateral	479,670,000
Fair Value of Collateral	479,670,000
5/26/2015 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		$ 450,000,000
Outstanding Amount		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
4/30/2015 | Term master repurchase agreement
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		$ 300,000,000
Outstanding Amount		174,853,000
Committed but Unfunded		125,147,000
Carrying Amount of Collateral		214,617,000
Fair Value of Collateral		214,617,000
Maturing on Various Date | Uncommitted securities repurchase facilities
Assets Sold under Agreements to Repurchase [Line Items]
Outstanding Amount		747,789,000
Carrying Amount of Collateral		861,456,000
Fair Value of Collateral		861,456,000
Maturing on Various Date | Mortgage loan financing
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement	555,785,756	447,409,690
Outstanding Amount	555,786,000	447,410,000
Committed but Unfunded	0	0
Carrying Amount of Collateral	723,407,000	591,613,000
Fair Value of Collateral	796,740,000	637,271,000
Maturing on Various Date | FHLB Financing
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement	2,248,711,953	1,900,000,000
Outstanding Amount	1,786,000,000	1,611,000,000
Committed but Unfunded	462,712,000	289,000,000
Carrying Amount of Collateral	2,250,547,000	2,068,988,000
Fair Value of Collateral	2,258,794,000	2,073,955,000
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
Outstanding Amount	20,400,000	11,000,000
Committed but Unfunded	29,600,000	$ 39,000,000
Interest Rate(s) (as a percent)		2.91%
Carrying Amount of Collateral	21,765,000	$ 0
Fair Value of Collateral	$ 22,920,000	0
10/6/2015 | Credit and Security Agreement
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		46,750,000
Outstanding Amount		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	1
Length of extension options	12 months	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
Outstanding Amount	50,000,000	25,000,000
Committed but Unfunded	$ 25,000,000	$ 50,000,000
Remaining Extension Options | period	2	2
Length of extension options	12 months	12 months
Minimum | 10/30/2016 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)	1.96%	2.42%
Minimum | 4/10/2016 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)	2.45%	2.41%
Minimum | 5/24/2016 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)	2.45%
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 | 10/2015 | Uncommitted securities repurchase facilities
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)	0.50%
Minimum | 5/26/2015 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)	2.45%	2.42%
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 | Maturing on Various Date | Uncommitted securities repurchase facilities
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		0.50%
Minimum | Maturing on Various Date | Mortgage loan financing
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)	4.25%	4.25%
Minimum | Maturing on Various Date | FHLB Financing
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)	0.27%	0.30%
Minimum | 1/24/2016 | Borrowings under credit agreement
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)	2.94%
Minimum | 2/11/2017 | Revolving credit facility
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)	3.69%	3.66%
Maximum | 10/30/2016 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)	2.71%	2.66%
Maximum | 4/10/2016 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)	4.21%	3.04%
Maximum | 5/24/2016 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)	4.21%
Maximum | 10/31/2016 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)	1.30%
Maximum | 10/2015 | Uncommitted securities repurchase facilities
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)	1.82%
Maximum | 5/26/2015 | Committed master repurchase agreements
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)	4.21%	3.16%
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 | Maturing on Various Date | Uncommitted securities repurchase facilities
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)		1.66%
Maximum | Maturing on Various Date | Mortgage loan financing
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)	6.75%	6.75%
Maximum | Maturing on Various Date | FHLB Financing
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)	2.74%	2.74%
Maximum | 1/24/2016 | Borrowings under credit agreement
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)	2.96%
Maximum | 2/11/2017 | Revolving credit facility
Assets Sold under Agreements to Repurchase [Line Items]
Interest Rate(s) (as a percent)	5.75%	5.75%