LOANS - Additional Information (Details) - USD ($)		3 Months Ended				12 Months Ended
		Mar. 31, 2019	Dec. 31, 2018		Mar. 31, 2018	Dec. 31, 2018
Mortgage Loans on Real Estate [Line Items]
Assets		$ 119,172,549,000	$ 105,787,527,000	[1]		$ 105,787,527,000	[1]
Loans receivable with variable rates of interest		90.00%				88.00%
Loan loss provision recorded during period		$ 5,703,000	3,500,000		$ 0
Balance of the loan loss provision		9,200,000	3,500,000			$ 3,500,000
Residential mortgages loans	[2]	3,879,324,000	4,585,975,000	[1]		4,585,975,000	[1]
Corporate Loans
Mortgage Loans on Real Estate [Line Items]
Residential mortgages loans		44,500,000	0			0
Commercial Mortgage Loan
Mortgage Loans on Real Estate [Line Items]
Residential mortgages loans		42,000,000	$ 42,200,000			$ 42,200,000
Residential Mortgage
Mortgage Loans on Real Estate [Line Items]
Assets		$ 1,300,000,000
Percent of adjustable-rate loans		45.00%	47.00%			47.00%
Residential mortgages loans		$ 101,300,000	$ 97,500,000			$ 97,500,000
Minimum
Mortgage Loans on Real Estate [Line Items]
Senior secured loans, stated maturity						5 years
Maximum
Mortgage Loans on Real Estate [Line Items]
Senior secured loans, stated maturity						7 years
Commercial Mortgage Loan
Mortgage Loans on Real Estate [Line Items]
Debt instrument face amount		$ 516,113,000	$ 516,834,000			$ 516,834,000

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

[2] (4) Includes $101.3 million and $97.5 million of residential mortgage loans held for sale, $42.0 million and $42.2 million of commercial mortgage loans held for sale and $44.5 million and $0 of corporate loans held for sale at March 31, 2019 and December 31, 2018, respectively.