LOANS - Additional Information (Details) - USD ($)		3 Months Ended		9 Months Ended		12 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Assets		$ 128,956,120,000		$ 128,956,120,000		$ 105,787,527,000	[1]
Loans receivable with variable rates of interest				91.00%		88.00%
Loan loss provision recorded during period		3,504,000	$ 0	$ 9,207,000	$ 0
Balance of the loan loss provision		12,700,000		12,700,000		$ 3,500,000
Loans	[2]	3,946,614,000		3,946,614,000		4,585,975,000	[1]
Commercial Mortgage Loan
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Loans		0	$ 0	0	$ 0	$ 42,200,000
Residential mortgage loans
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Assets		$ 1,200,000,000		$ 1,200,000,000
Percent of adjustable-rate loans		38.00%		38.00%		47.00%
Loans		$ 76,300,000		$ 76,300,000		$ 97,500,000
Minimum
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Senior secured loans, stated maturity				5 years
Maximum
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Senior secured loans, stated maturity				7 years
Commercial Mortgage Loan
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Mortgage Principal		$ 491,435,000		$ 491,435,000		$ 516,834,000

[1] ⁽¹⁾  Derived from the audited consolidated financial statements at December 31, 2018.

[2] ⁽⁴⁾  Includes $76.3 million and $97.5 million of residential mortgage loans held for sale and $0 and $42.2 million of commercial mortgage loans held for sale at September 30, 2019 and December 31, 2018, respectively.