MORTGAGE LOAN RECEIVABLES - Additional Information (Details)		3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 29, 2017 USD ($) loan	Jun. 30, 2018 USD ($) loan	Mar. 31, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) loan	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($) loan
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Proceeds from sales of mortgage loan receivables	$ 625,700,000
Outstanding face amount		$ 3,896,886,000			$ 3,896,886,000		$ 3,533,236,000
Risk retention requirement, amount	$ 12,900,000
Risk retention requirement, percentage	2.00%
Sold, horizontal interest percentage	3.00%
Controlling classes, percentage	98.00%
Period of risk retention requirement to be held (at least)	5 years
Purchase, not recognized for accounting purposes	$ 62,700,000
Gain on sale	26,100,000
Liability for transfers not considered sales	$ 78,800,000
Unamortized discounts included in mortgage loan receivables held for investment, at amortized cost		$ 25,300			$ 25,300		$ 200,000
Number of mortgage loans impaired | loan		0			0		0
Provision for loan losses		$ 300,000		$ 0	$ 3,300,000	$ 0
Provision for loss resulting from on-going bankruptcy proceedings			$ 2,700,000
First mortgage loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of loans | loan	34
Outstanding face amount	$ 548,936,189	3,630,000,000			3,630,000,000		$ 3,140,788,000
Carrying value	$ 547,700,000
Intercompany loans held-for-investment
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of loans | loan	23
Outstanding face amount	$ 76,717,000
Mortgage loan receivables held for investment, net, at amortized cost
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Outstanding face amount		3,788,546,000			3,788,546,000		3,300,709,000
Carrying value		3,764,172,000			3,764,172,000		3,282,462,000
Loans receivable with fixed rates of interest		$ 781,200,000			$ 781,200,000		$ 723,700,000
Percentage of loans receivable with fixed rates of interest		20.80%			20.80%		22.00%
Loans receivable with variable rates of interest		$ 3,000,000,000			$ 3,000,000,000		$ 2,600,000,000
Loans receivable with variable rates of interest, percentage		79.20%			79.20%		78.00%
Number or loans in default | loan					2
Reserve based on targeted percentage level in portfolio		$ 600,000			$ 600,000
Loans in default, carrying value		26,900,000			26,900,000
Mortgage loan receivables held for sale
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Outstanding face amount		108,340,000			108,340,000		$ 232,527,000
Loans receivable with fixed rates of interest		$ 107,700,000			$ 107,700,000		$ 230,200,000
Percentage of loans receivable with fixed rates of interest		100.00%			100.00%		100.00%
Loan on non-accrual status
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loans nonaccrual status, amount							$ 0
Minimum
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Direct capitalization rate			4.70%		4.70%		4.35%
Maximum
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Direct capitalization rate			5.00%		5.00%		4.65%