DEBT OBLIGATIONS, NET - Schedule of Company's Debt Obligations (Details)		3 Months Ended	12 Months Ended
	Mar. 21, 2017 Extension	Mar. 31, 2018 USD ($) Extension	Dec. 31, 2017 USD ($) Extension	Dec. 21, 2017 USD ($)	Oct. 17, 2017 USD ($)	Feb. 22, 2017 USD ($)
Assets Sold under Agreements to Repurchase [Line Items]
Debt Obligations Outstanding		$ 754,377,000	$ 473,410,000
Debt obligations		4,624,608,000
Carrying Amount of Collateral		0	0
Committed Loan Repurchase Facility
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		1,650,000,000	1,650,000,000
Debt Obligations Outstanding		550,523,000	398,653,000
Committed but Unfunded		1,099,477,000	1,251,347,000
Carrying Amount of Collateral		914,280,000	667,102,000
Fair Value of Collateral		914,703,000	668,664,000
Committed Loan Repurchase Facility | 10/1/2020
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		600,000,000	600,000,000
Debt Obligations Outstanding		212,023,000	120,493,000
Committed but Unfunded		387,977,000	479,507,000
Carrying Amount of Collateral		348,473,000	160,031,000
Fair Value of Collateral		$ 347,019,000	$ 159,568,000
Number of extension maturity periods | Extension		2	2
Length of extension options		12 months	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		3.53%	3.23%
Committed Loan Repurchase Facility | 10/1/2020 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		4.28%	3.98%
Committed Loan Repurchase Facility | 5/24/2018
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		$ 450,000,000	$ 450,000,000
Debt Obligations Outstanding		154,077,000	183,111,000
Committed but Unfunded		295,923,000	266,889,000
Carrying Amount of Collateral		280,892,000	333,647,000
Fair Value of Collateral		$ 282,231,000	$ 335,076,000
Number of extension maturity periods | Extension		3	3
Length of extension options		12 months	12 months
Committed Loan Repurchase Facility | 5/24/2018 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		3.93%	3.63%
Committed Loan Repurchase Facility | 5/24/2018 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		4.78%	4.48%
Committed Loan Repurchase Facility | 4/7/2019
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		$ 300,000,000
Debt Obligations Outstanding		84,687,000
Committed but Unfunded		215,313,000
Carrying Amount of Collateral		137,421,000
Fair Value of Collateral		$ 137,983,000
Number of extension maturity periods | Extension		1
Number of additional extension maturity periods | Extension		1
Length of additional extension maturity periods		364 days
Committed Loan Repurchase Facility | 4/10/2018
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing			$ 300,000,000			$ 300,000,000.0
Debt Obligations Outstanding			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
Length of extension options			364 days
Additional length of period of extension options			364 days
Number of additional extension maturity periods | Extension	1
Length of additional extension maturity periods	364 days
Committed Loan Repurchase Facility | 4/10/2018 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		3.78%	3.73%
Committed Loan Repurchase Facility | 4/10/2018 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		5.03%	4.73%
Committed Loan Repurchase Facility | 2/29/2020
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		$ 200,000,000	$ 200,000,000
Debt Obligations Outstanding		80,536,000	32,042,000
Committed but Unfunded		119,464,000	167,958,000
Carrying Amount of Collateral		122,123,000	48,045,000
Fair Value of Collateral		$ 122,099,000	$ 48,045,000
Number of extension maturity periods | Extension		1	1
Length of extension options		12 months	12 months
Number of additional extension maturity periods | Extension		2	2
Length of additional extension maturity periods		6 months	6 months
Committed Loan Repurchase Facility | 2/29/2020 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		4.31%	4.25%
Committed Loan Repurchase Facility | 2/29/2020 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		4.81%	4.50%
Committed Loan Repurchase Facility | 6/28/2019
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		$ 100,000,000	$ 100,000,000
Debt Obligations Outstanding		19,200,000	0
Committed but Unfunded		$ 80,800,000	100,000,000
Interest rate		4.28%
Carrying Amount of Collateral		$ 25,371,000	0
Fair Value of Collateral		25,371,000	0
Committed Securities Repurchase Facility
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		400,000,000
Committed Securities Repurchase Facility | 9/30/2019
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		400,000,000	400,000,000
Debt Obligations Outstanding		98,762,000	0
Committed but Unfunded		301,238,000	400,000,000
Carrying Amount of Collateral		119,600,000	0
Fair Value of Collateral		$ 119,600,000	0
Committed Securities Repurchase Facility | 9/30/2019 | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		2.36%
Committed Securities Repurchase Facility | 9/30/2019 | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		2.56%
Uncommitted Securities Repurchase Facility | Various Dates
Assets Sold under Agreements to Repurchase [Line Items]
Debt Obligations Outstanding		$ 105,092,000	74,757,000
Carrying Amount of Collateral		119,969,000	86,322,000
Fair Value of Collateral		119,969,000	86,322,000
Purchase, not reflected in consolidated financial statement		2,600,000	$ 26,700,000
Securities phased in secondary market		$ 6,000,000
Uncommitted Securities Repurchase Facility | Various Dates | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		2.39%	1.65%
Uncommitted Securities Repurchase Facility | Various Dates | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		3.69%	3.31%
Total Repurchase Facilities
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		$ 2,050,000,000	$ 2,050,000,000
Debt Obligations Outstanding		754,377,000	473,410,000
Committed but Unfunded		1,400,715,000	1,651,347,000
Carrying Amount of Collateral		1,153,849,000	753,424,000
Fair Value of Collateral		1,154,272,000	754,986,000
Revolving Credit Facility | 2/11/2019
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		241,430,000	241,430,000
Debt Obligations Outstanding		0	0
Committed but Unfunded		241,430,000	241,430,000
Mortgage Loan Financing | Various Dates
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		683,487,000	692,696,000
Debt Obligations Outstanding		683,487,000	692,696,000
Committed but Unfunded		0	0
Carrying Amount of Collateral		895,786,000	911,034,000
Fair Value of Collateral		$ 1,056,848,000	$ 1,066,708,000
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 Financing		$ 683,095,000	$ 688,479,000
Debt Obligations Outstanding		683,095,000	688,479,000
Committed but Unfunded		0	0
Carrying Amount of Collateral		881,719,000	880,385,000	$ 431,500,000	$ 456,900,000
Fair Value of Collateral		882,114,000	881,576,000
Unamortized debt issuance costs		$ 5,400,000	$ 6,000,000
CLO Debt | Various Dates | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		2.67%	2.36%
CLO Debt | Various Dates | Maximum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		5.39%	5.08%
Participation Financing - Mortgage Loan Receivable
Assets Sold under Agreements to Repurchase [Line Items]
Debt Obligations Outstanding			$ 2,800,000
Participation Financing - Mortgage Loan Receivable | 6/6/2018
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		$ 2,815,000	3,107,000
Debt Obligations Outstanding		2,815,000	3,107,000
Committed but Unfunded		$ 0	$ 0
Stated interest rate on debt instrument (as a percent)		17.00%	17.00%
Carrying Amount of Collateral		$ 2,815,000	$ 3,107,000
Fair Value of Collateral		2,815,000	3,107,000
Borrowings from the FHLB | Various Dates
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		1,933,522,000	2,000,000,000
Debt Obligations Outstanding		1,348,000,000	1,370,000,000
Committed but Unfunded		585,522,000	630,000,000
Carrying Amount of Collateral		1,827,338,000	1,777,597,000
Fair Value of Collateral		1,830,545,000	1,783,210,000
Purchase, not reflected in consolidated financial statement		$ 9,900,000	$ 10,100,000
Borrowings from the FHLB | Various Dates | Minimum
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		0.87%	0.87%
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		$ 13,366,000
Senior Unsecured Notes | Various Dates
Assets Sold under Agreements to Repurchase [Line Items]
Debt issued		1,166,201,000	$ 1,166,201,000
Senior Unsecured Notes		1,152,834,000	1,152,134,000
Committed but Unfunded		0	0
Unamortized debt issuance costs		$ 13,400,000	$ 14,100,000
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%	5.25%
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%	5.875%
Total Debt Obligations
Assets Sold under Agreements to Repurchase [Line Items]
Debt issued		$ 6,760,550,000	$ 6,841,913,000
Debt obligations		4,624,608,000	4,379,826,000
Committed but Unfunded		2,227,667,000	2,522,777,000
Carrying Amount of Collateral		4,761,507,000	4,325,547,000
Fair Value of Collateral		$ 4,926,594,000	$ 4,489,587,000