DEBT OBLIGATIONS, NET - Schedule of Company's Debt Obligations (Details)		6 Months Ended	12 Months Ended
	Jan. 04, 2018	Jun. 30, 2019 USD ($) Extension	Dec. 31, 2018 USD ($) Extension	Dec. 27, 2018 USD ($)	Dec. 21, 2017 USD ($)	Oct. 17, 2017 USD ($)
Assets Sold under Agreements to Repurchase [Line Items]
Debt Obligations Outstanding		$ 1,267,371,000	$ 663,685,000
Debt obligations		4,613,088,000
Carrying Amount of Collateral		0	0
Committed Loan Repurchase Facility
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		1,750,000,000	1,750,000,000	$ 100,000,000.0
Debt Obligations Outstanding		685,260,000	497,531,000
Committed but Unfunded		1,064,740,000	1,252,469,000
Carrying Amount of Collateral		1,044,667,000	744,310,000
Fair Value of Collateral		1,047,142,000	745,103,000
Length of additional extension maturity periods	1 year
Committed Loan Repurchase Facility | 2/24/2022
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		600,000,000
Debt Obligations Outstanding		200,501,000
Committed but Unfunded		399,499,000
Carrying Amount of Collateral		282,722,000
Fair Value of Collateral		$ 282,798,000
Number of extension maturity periods | Extension		2
Length of extension options		12 months
Committed Loan Repurchase Facility | 5/24/2020
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		$ 350,000,000
Debt Obligations Outstanding		63,996,000
Committed but Unfunded		286,004,000
Carrying Amount of Collateral		98,515,000
Fair Value of Collateral		$ 100,652,000
Number of extension maturity periods | Extension		1
Length of extension options		12 months
Committed Loan Repurchase Facility | 5/24/2019
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing			350,000,000
Debt Obligations Outstanding			63,679,000
Committed but Unfunded			286,321,000
Carrying Amount of Collateral			87,385,000
Fair Value of Collateral			88,762,000
Committed Loan Repurchase Facility | 4/10/2020
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		$ 300,000,000
Debt Obligations Outstanding		177,710,000
Committed but Unfunded		122,290,000
Carrying Amount of Collateral		302,349,000
Fair Value of Collateral		$ 302,349,000
Number of extension maturity periods | Extension		1
Length of extension options		364 days
Committed Loan Repurchase Facility | 5/6/2021
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		$ 300,000,000	300,000,000
Debt Obligations Outstanding		102,499,000	79,886,000
Committed but Unfunded		197,501,000	220,114,000
Carrying Amount of Collateral		155,332,000	117,382,000
Fair Value of Collateral		$ 155,332,000	$ 117,366,000
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 | 7/20/2021
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		$ 100,000,000	$ 100,000,000
Debt Obligations Outstanding		87,174,000	52,738,000
Committed but Unfunded		12,826,000	47,262,000
Carrying Amount of Collateral		133,949,000	72,154,000
Fair Value of Collateral		$ 134,211,000	$ 72,154,000
Number of extension maturity periods | Extension		1	1
Length of extension options		12 months	12 months
Committed Loan Repurchase Facility | 3/26/2020
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		$ 100,000,000
Debt Obligations Outstanding		53,380,000
Committed but Unfunded		$ 46,620,000
Interest rate		4.39%
Carrying Amount of Collateral		$ 71,800,000
Fair Value of Collateral		$ 71,800,000
Committed Loan Repurchase Facility | 12/26/2019
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing			$ 100,000,000
Debt Obligations Outstanding			0
Committed but Unfunded			100,000,000
Carrying Amount of Collateral			0
Fair Value of Collateral			0
Committed Loan Repurchase Facility | 4/7/2019
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing			300,000,000
Debt Obligations Outstanding			120,631,000
Committed but Unfunded			179,369,000
Carrying Amount of Collateral			204,747,000
Fair Value of Collateral			$ 205,219,000
Number of extension maturity periods | Extension			1
Length of extension options			364 days
Number of additional extension maturity periods | Extension			1
Additional length of period of extension options			364 days
Committed Loan Repurchase Facility | 10/1/2020
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing			$ 600,000,000
Debt Obligations Outstanding			180,597,000
Committed but Unfunded			419,403,000
Carrying Amount of Collateral			262,642,000
Fair Value of Collateral			$ 261,602,000
Number of extension maturity periods | Extension			2
Length of extension options			12 months
Committed Loan Repurchase Facility | 2/11/2020
Assets Sold under Agreements to Repurchase [Line Items]
Length of extension options		12 months
Committed Securities Repurchase Facility
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		$ 400,000,000.0
Committed Securities Repurchase Facility | 3/4/2021
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		400,000,000
Debt Obligations Outstanding		0
Committed but Unfunded		400,000,000
Carrying Amount of Collateral		0
Fair Value of Collateral		0
Committed Securities Repurchase Facility | 9/30/2019
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing			$ 400,000,000
Debt Obligations Outstanding			0
Committed but Unfunded			400,000,000
Carrying Amount of Collateral			0
Fair Value of Collateral			0
Uncommitted Securities Repurchase Facility | Various Date
Assets Sold under Agreements to Repurchase [Line Items]
Debt Obligations Outstanding		582,111,000	166,154,000
Carrying Amount of Collateral		637,238,000	187,803,000
Fair Value of Collateral		637,238,000	187,803,000
Purchase, not reflected in consolidated financial statement		3,000,000.0	3,000,000.0
Securities phased in secondary market			6,000,000.0
Total Repurchase Facilities
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		2,150,000,000	2,150,000,000
Debt Obligations Outstanding		1,267,371,000	663,685,000
Committed but Unfunded		1,464,740,000	1,652,469,000
Carrying Amount of Collateral		1,681,905,000	932,113,000
Fair Value of Collateral		1,684,380,000	$ 932,906,000
Revolving Credit Facility | 5/24/2019
Assets Sold under Agreements to Repurchase [Line Items]
Number of extension maturity periods | Extension			2
Length of extension options			12 months
Revolving Credit Facility | 2/11/2020
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		266,430,000
Debt Obligations Outstanding		0
Committed but Unfunded		$ 266,430,000
Number of extension maturity periods | Extension		3
Revolving Credit Facility | 2/11/2019
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing			$ 266,430,000
Debt Obligations Outstanding			0
Committed but Unfunded			$ 266,430,000
Number of extension maturity periods | Extension			4
Length of extension options			12 months
Mortgage Loan Financing | Various Date
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		$ 734,652,000	$ 743,902,000
Debt Obligations Outstanding		734,652,000	743,902,000
Committed but Unfunded		0	0
Carrying Amount of Collateral		912,888,000	939,362,000
Fair Value of Collateral		1,097,090,000	1,108,968,000
CLO Debt | Various Date
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		263,216,000	601,543,000
Debt Obligations Outstanding		263,216,000	601,543,000
Committed but Unfunded		0	0
Carrying Amount of Collateral		425,439,000	710,502,000		$ 431,500,000	$ 456,900,000
Fair Value of Collateral		425,810,000	710,737,000
Unamortized debt issuance costs		1,000,000.0	2,600,000
Participation Financing - Mortgage Loan Receivable
Assets Sold under Agreements to Repurchase [Line Items]
Debt Obligations Outstanding		0	2,500,000
Participation Financing - Mortgage Loan Receivable | 6/6/2019
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing			2,453,000
Debt Obligations Outstanding			2,453,000
Committed but Unfunded			$ 0
Interest rate			17.00%
Carrying Amount of Collateral			$ 2,453,000
Fair Value of Collateral			2,453,000
Borrowings from the FHLB | Various Date
Assets Sold under Agreements to Repurchase [Line Items]
Committed Financing		1,945,795,000	1,933,522,000
Debt Obligations Outstanding		1,191,449,000	1,286,000,000
Committed but Unfunded		754,346,000	647,522,000
Carrying Amount of Collateral		1,493,752,000	1,652,952,000
Fair Value of Collateral		1,500,648,000	1,655,150,000
Purchase, not reflected in consolidated financial statement		10,000,000.0	9,700,000
Senior Unsecured Notes
Assets Sold under Agreements to Repurchase [Line Items]
Unamortized debt issuance costs		9,801,000
Senior Unsecured Notes | Various Date
Assets Sold under Agreements to Repurchase [Line Items]
Debt issued		1,166,201,000	1,166,201,000
Senior Unsecured Notes		1,156,400,000	1,154,991,000
Committed but Unfunded		0	0
Unamortized debt issuance costs		9,800,000	11,200,000
Total Debt Obligations
Assets Sold under Agreements to Repurchase [Line Items]
Debt issued		6,526,294,000	6,864,051,000
Debt obligations		4,613,088,000	4,452,574,000
Committed but Unfunded		2,485,516,000	2,566,421,000
Carrying Amount of Collateral		4,513,984,000	4,237,382,000
Fair Value of Collateral		$ 4,707,928,000	$ 4,410,214,000
Minimum | Committed Loan Repurchase Facility | 2/24/2022
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		4.14%
Minimum | Committed Loan Repurchase Facility | 5/24/2020
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		4.62%
Minimum | Committed Loan Repurchase Facility | 5/24/2019
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate			4.68%
Minimum | Committed Loan Repurchase Facility | 4/10/2020
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		4.39%
Minimum | Committed Loan Repurchase Facility | 5/6/2021
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		4.44%	4.44%
Minimum | Committed Loan Repurchase Facility | 7/20/2021
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		4.21%	4.58%
Minimum | Committed Loan Repurchase Facility | 4/7/2019
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate			4.46%
Minimum | Committed Loan Repurchase Facility | 10/1/2020
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate			4.21%
Minimum | Uncommitted Securities Repurchase Facility | Various Date
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		2.99%	2.99%
Minimum | Mortgage Loan Financing | Various Date
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		4.25%	4.25%
Minimum | CLO Debt | Various Date
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		3.40%	3.34%
Minimum | Borrowings from the FHLB | Various Date
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		1.47%	1.18%
Minimum | Senior Unsecured Notes | Various Date
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		5.25%	5.25%
Maximum | Committed Loan Repurchase Facility | 2/24/2022
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		4.64%
Maximum | Committed Loan Repurchase Facility | 5/24/2020
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		4.97%
Maximum | Committed Loan Repurchase Facility | 5/24/2019
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate			4.98%
Maximum | Committed Loan Repurchase Facility | 4/10/2020
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		4.90%
Maximum | Committed Loan Repurchase Facility | 5/6/2021
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		4.94%	4.94%
Maximum | Committed Loan Repurchase Facility | 7/20/2021
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		4.65%	4.96%
Maximum | Committed Loan Repurchase Facility | 4/7/2019
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate			4.96%
Maximum | Committed Loan Repurchase Facility | 10/1/2020
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate			4.96%
Maximum | Uncommitted Securities Repurchase Facility | Various Date
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		4.06%	4.55%
Maximum | Mortgage Loan Financing | Various Date
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		7.00%	7.00%
Maximum | CLO Debt | Various Date
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		5.99%	6.06%
Maximum | Borrowings from the FHLB | Various Date
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		3.00%	3.01%
Maximum | Senior Unsecured Notes | Various Date
Assets Sold under Agreements to Repurchase [Line Items]
Interest rate		5.875%	5.875%