DEBT OBLIGATIONS, NET - Schedule of Company's Debt Obligations (Details)		12 Months Ended
	Mar. 21, 2017 Extension	Dec. 31, 2017 USD ($) Extension	Dec. 31, 2016 USD ($) Extension	Dec. 21, 2017 USD ($)	Oct. 17, 2017 USD ($)	Feb. 22, 2017 USD ($)	Aug. 03, 2016 USD ($) Extension	Jul. 01, 2016 USD ($)	Jun. 28, 2016 USD ($)	Apr. 19, 2016 Extension
Assets Sold under Agreements to Repurchase [Line Items]
Gross amounts of recognized liabilities		$ 473,410,000	$ 1,107,185,000
Secured and unsecured debt obligations		4,379,826,000
Carrying Amount of Collateral		0	0
Committed Loan Repurchase Facility
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		1,650,000,000	1,650,000,000
Gross amounts of recognized liabilities		398,653,000	567,163,000
Committed but Unfunded		1,251,347,000	1,082,837,000
Carrying Amount of Collateral		667,102,000	961,791,000
Fair Value of Collateral		668,664,000	965,888,000
Committed Loan Repurchase Facility | 10/1/2020
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		600,000,000
Gross amounts of recognized liabilities		120,493,000
Committed but Unfunded		479,507,000
Carrying Amount of Collateral		160,031,000
Fair Value of Collateral		$ 159,568,000
Number of extension maturity periods | Extension		2
Length of extension options		12 months
Length of additional extension maturity periods	12 months
Committed Loan Repurchase Facility | 10/1/2020 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		3.23%
Committed Loan Repurchase Facility | 10/1/2020 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		3.98%
Committed Loan Repurchase Facility | 10/30/2018
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement			600,000,000
Gross amounts of recognized liabilities			183,604,000
Committed but Unfunded			416,396,000
Carrying Amount of Collateral			292,628,000
Fair Value of Collateral			$ 293,618,000
Number of extension maturity periods | Extension			3
Length of extension options			12 months
Committed Loan Repurchase Facility | 10/30/2018 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate			2.45%
Committed Loan Repurchase Facility | 10/30/2018 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate			3.27%
Committed Loan Repurchase Facility | 5/24/2018
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		$ 450,000,000
Gross amounts of recognized liabilities		183,111,000
Committed but Unfunded		266,889,000
Carrying Amount of Collateral		333,647,000
Fair Value of Collateral		$ 335,076,000
Number of extension maturity periods | Extension		3								2
Length of extension options		12 months
Committed Loan Repurchase Facility | 5/24/2018 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		3.63%
Committed Loan Repurchase Facility | 5/24/2018 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		4.48%
Committed Loan Repurchase Facility | 5/24/2017
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement			$ 450,000,000
Gross amounts of recognized liabilities			184,158,000
Committed but Unfunded			265,842,000
Carrying Amount of Collateral			286,848,000
Fair Value of Collateral			$ 288,267,000
Number of extension maturity periods | Extension			3
Length of extension options			12 months
Committed Loan Repurchase Facility | 5/24/2017 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate			2.95%
Committed Loan Repurchase Facility | 5/24/2017 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate			3.70%
Committed Loan Repurchase Facility | 4/10/2018
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		$ 300,000,000				$ 300,000,000.0
Gross amounts of recognized liabilities		63,007,000
Committed but Unfunded		236,993,000
Carrying Amount of Collateral		125,379,000
Fair Value of Collateral		$ 125,975,000
Number of extension maturity periods | Extension		2
Number of additional extension maturity periods | Extension	1	1
Length of additional extension maturity periods	364 days	364 days
Committed Loan Repurchase Facility | 4/10/2018 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		3.73%
Committed Loan Repurchase Facility | 4/10/2018 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		4.73%
Committed Loan Repurchase Facility | 4/9/2017
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement			$ 400,000,000
Gross amounts of recognized liabilities			100,979,000
Committed but Unfunded			299,021,000
Carrying Amount of Collateral			235,878,000
Fair Value of Collateral			$ 236,696,000
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			2.95%
Committed Loan Repurchase Facility | 4/9/2017 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate			3.99%
Committed Loan Repurchase Facility | 2/29/2020
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		$ 200,000,000
Gross amounts of recognized liabilities		32,042,000
Committed but Unfunded		167,958,000
Carrying Amount of Collateral		48,045,000
Fair Value of Collateral		$ 48,045,000
Number of extension maturity periods | Extension		1
Length of extension options		12 months
Number of additional extension maturity periods | Extension		2
Length of additional extension maturity periods		6 months
Committed Loan Repurchase Facility | 2/29/2020 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		4.25%
Committed Loan Repurchase Facility | 2/29/2020 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		4.50%
Committed Loan Repurchase Facility | 6/28/2019
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		$ 100,000,000	$ 100,000,000						$ 100,000,000
Gross amounts of recognized liabilities		0	27,132,000
Committed but Unfunded		100,000,000	72,868,000
Carrying Amount of Collateral		0	36,166,000
Fair Value of Collateral		0	$ 36,410,000
Committed Loan Repurchase Facility | 6/28/2019 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate			2.90%
Committed Loan Repurchase Facility | 6/28/2019 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate			3.13%
Committed Loan Repurchase Facility | 8/2/2019
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement			$ 100,000,000				$ 100,000,000
Gross amounts of recognized liabilities			71,290,000
Committed but Unfunded			28,710,000
Carrying Amount of Collateral			110,271,000
Fair Value of Collateral			$ 110,897,000
Number of extension maturity periods | Extension			1				1
Length of extension options			12 months
Number of additional extension maturity periods | Extension			2				2
Length of additional extension maturity periods			6 months
Committed Loan Repurchase Facility | 8/2/2019 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate			2.93%
Committed Loan Repurchase Facility | 8/2/2019 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate			3.68%
Committed Securities Repurchase Facility
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		400,000,000
Committed Securities Repurchase Facility | 9/30/2019
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		400,000,000
Gross amounts of recognized liabilities		0
Committed but Unfunded		400,000,000
Carrying Amount of Collateral		0
Fair Value of Collateral		0
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
Gross amounts of recognized liabilities			228,317,000
Committed but Unfunded			171,683,000
Carrying Amount of Collateral			272,402,000
Fair Value of Collateral			$ 272,402,000
Committed Securities Repurchase Facility | 7/1/2018 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate			1.00%
Committed Securities Repurchase Facility | 7/1/2018 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate			2.59%
Uncommitted Securities Repurchase Facility | Various Dates
Assets Sold under Agreements to Repurchase [Line Items]
Gross amounts of recognized liabilities		74,757,000	$ 311,705,000
Carrying Amount of Collateral		86,322,000	368,638,000
Fair Value of Collateral		86,322,000	$ 368,638,000
Purchase, not reflected in consolidated financial statement		$ 26,700,000
Uncommitted Securities Repurchase Facility | Various Dates | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		1.65%	1.00%
Uncommitted Securities Repurchase Facility | Various Dates | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		3.31%	2.41%
Total Repurchase Facilities
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		$ 2,050,000,000	$ 2,050,000,000
Gross amounts of recognized liabilities		473,410,000	1,107,185,000
Committed but Unfunded		1,651,347,000	1,254,520,000
Carrying Amount of Collateral		753,424,000	1,602,831,000
Fair Value of Collateral		754,986,000	1,606,928,000
Revolving Credit Facility | 2/11/2018
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		241,430,000
Gross amounts of recognized liabilities		0
Committed but Unfunded		241,430,000
Revolving Credit Facility | 2/11/2017
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement			143,000,000
Gross amounts of recognized liabilities			25,000,000
Committed but Unfunded			$ 118,000,000
Interest rate			3.16%
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		692,696,000	$ 590,106,000
Gross amounts of recognized liabilities		692,696,000	590,106,000
Committed but Unfunded			0
Carrying Amount of Collateral		911,034,000	757,468,000
Fair Value of Collateral		$ 1,066,708,000	$ 875,160,000
Mortgage Loan Financing | Various Dates | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		4.25%	4.25%
Mortgage Loan Financing | Various Dates | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		6.75%	6.75%
CLO Debt | Various Dates
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		$ 688,479,000
Gross amounts of recognized liabilities		688,479,000
Committed but Unfunded		0
Carrying Amount of Collateral		880,385,000		$ 431,500,000	$ 456,900,000
Fair Value of Collateral		881,576,000
Unamortized debt issuance costs		$ 6,000,000
CLO Debt | Various Dates | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		2.36%
CLO Debt | Various Dates | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		5.08%
Participation Financing - Mortgage Loan Receivable | 6/6/2018
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		$ 3,107,000
Gross amounts of recognized liabilities		$ 3,107,000
Stated interest rate on debt instrument (as a percent)		17.00%
Carrying Amount of Collateral		$ 3,107,000
Fair Value of Collateral		3,107,000
Borrowings from the FHLB | Various Dates
Assets Sold under Agreements to Repurchase [Line Items]
Committed amount on master repurchase agreement		2,000,000,000	$ 1,998,931,000
Gross amounts of recognized liabilities		1,370,000,000	1,660,000,000
Committed but Unfunded		630,000,000	338,931,000
Carrying Amount of Collateral		1,777,597,000	2,162,779,000
Fair Value of Collateral		1,783,210,000	$ 2,167,017,000
Purchase, not reflected in consolidated financial statement		$ 10,100,000
Borrowings from the FHLB | Various Dates | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		0.87%	0.43%
Borrowings from the FHLB | Various Dates | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		2.74%	2.74%
Senior Unsecured Notes
Assets Sold under Agreements to Repurchase [Line Items]
Unamortized debt issuance costs		$ 14,067,000
Senior Unsecured Notes | Various Dates
Assets Sold under Agreements to Repurchase [Line Items]
Debt issued		1,166,201,000	$ 563,872,000
Senior unsecured notes		1,152,134,000	559,847,000
Committed but Unfunded		0	0
Unamortized debt issuance costs		$ 14,100,000	$ 4,000,000
Senior Unsecured Notes | Various Dates | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Stated interest rate on debt instrument (as a percent)		5.25%	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)		5.875%	7.375%
Total Debt Obligations
Assets Sold under Agreements to Repurchase [Line Items]
Debt issued		$ 6,841,913,000	$ 5,345,909,000
Secured and unsecured debt obligations		4,379,826,000	3,942,138,000
Committed but Unfunded		2,522,777,000	1,711,451,000
Carrying Amount of Collateral		4,325,547,000	4,523,078,000
Fair Value of Collateral		$ 4,489,587,000	$ 4,649,105,000