MORTGAGE LOAN RECEIVABLES - Additional Information (Details)			3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($) loan	Jun. 29, 2017 USD ($) loan	Sep. 30, 2017 USD ($) loan	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) loan	Sep. 30, 2016 USD ($) loan	Dec. 31, 2016 USD ($) loan
Mortgage Loans on Real Estate [Line Items]
Proceeds from sales of mortgage loan receivables		$ 625,700,000
Outstanding Face Amount	$ 3,989,046,000		$ 3,989,046,000		$ 3,989,046,000		$ 2,371,827,000
Risk retention requirement, amount		$ 12,900,000
Risk retention requirement, percentage		2.00%
Sold, horizontal interest percentage		3.00%
Controlling classes, percentage		98.00%
Purchase, not recognized for accounting purposes		$ 62,700,000
Interest income			72,763,000	$ 60,284,000	196,410,000	$ 175,650,000
Liability for transfers not considered sales	631,480,000		631,480,000		631,480,000		0
Proceeds from sales of real estate securities					983,386,000	308,429,000
Interest expense			42,607,000	$ 30,685,000	109,625,000	$ 88,622,000
Unamortized discounts included in mortgage loan receivables held for investment, at amortized cost	$ 2,300,000		$ 2,300,000		$ 2,300,000		$ 600,000
Number of mortgage loans impaired | loan	0		0		0		0
Accrued and unpaid interest receivable	$ 26,426,000		$ 26,426,000		$ 26,426,000		$ 24,439,000
First mortgage loan, held for investment
Mortgage Loans on Real Estate [Line Items]
Number of loans | loan	34	34
Outstanding Face Amount	$ 2,703,936,000	$ 549,000,000.0	2,703,936,000		2,703,936,000		1,843,006,000
Carrying Value	546,700,000	$ 547,700,000	546,700,000		546,700,000
Carrying value	2,688,845,000		2,688,845,000		2,688,845,000		1,832,626,000
Non-Controlling loan interest
Mortgage Loans on Real Estate [Line Items]
Number of loans | loan		1
Outstanding Face Amount		$ 52,300,000
Carrying Value		51,800,000
Liability for transfers not considered sales		$ 52,100,000
Intercompany Loans Held-For-Investment
Mortgage Loans on Real Estate [Line Items]
Number of loans | loan		23
Outstanding Face Amount		$ 76,700,000
Mortgage loans transferred but not considered sold
Mortgage Loans on Real Estate [Line Items]
Outstanding Face Amount	600,222,000		600,222,000		600,222,000		0
Carrying Value	598,525,000		598,525,000		598,525,000
Carrying value	598,525,000		598,525,000		598,525,000		0
Interest income			7,200,000		7,300,000
Liability for transfers not considered sales		580,000,000.0
Proceeds from sales of real estate securities		655,600,000
Purchase, not reflected in consolidated financial statement		$ 75,600,000
Interest expense			6,200,000		6,300,000
Loans receivable with fixed rates of interest	598,500,000		598,500,000		598,500,000
Mortgage loan receivables held for investment, net, at amortized cost
Mortgage Loans on Real Estate [Line Items]
Outstanding Face Amount	3,462,961,000		3,462,961,000		3,462,961,000		2,011,309,000
Carrying value	3,445,465,000		3,445,465,000		3,445,465,000		2,000,095,000
Loans receivable with fixed rates of interest	$ 1,200,000,000		$ 1,200,000,000		$ 1,200,000,000		$ 205,400,000
Percentage of loans receivable with fixed rates of interest	34.70%		34.70%		34.70%		10.30%
Loans receivable with variable rates of interest	$ 2,200,000,000		$ 2,200,000,000		$ 2,200,000,000		$ 1,800,000,000
Loans receivable with variable rates of interest, percentage	65.30%		65.30%		65.30%		89.70%
Number or loans in default | loan					2	2
Loans in default, carrying value	$ 26,900,000		$ 26,900,000		$ 26,900,000
Impairment recorded on defaulted loans					0
Accrued and unpaid interest receivable	4,800,000		4,800,000		4,800,000		$ 3,500,000
Default interest included in accrued and unpaid interest receivable	3,500,000		3,500,000		3,500,000		2,200,000
Mortgage loan receivables held for sale
Mortgage Loans on Real Estate [Line Items]
Outstanding Face Amount	526,085,000		526,085,000		526,085,000		360,518,000
Loans receivable with fixed rates of interest	$ 523,000,000.0		$ 523,000,000.0		$ 523,000,000.0		$ 360,500,000
Percentage of loans receivable with fixed rates of interest	100.00%		100.00%		100.00%		100.00%
Loan on non-accrual status
Mortgage Loans on Real Estate [Line Items]
Loans nonaccrual status, amount	$ 0		$ 0		$ 0		$ 0