SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Schedule Of Significant Accounting Policies [Line Items]
Real estate acquired in settlement of residential mortgage loans	$ 0		$ 0
Other-than-temporary impairment recognized	0	$ 0
Outstanding impaired loans, nonaccrual loans, and loans in default	0		0
Allowance for loan losses	$ 0		0
Minimum
Schedule Of Significant Accounting Policies [Line Items]
Senior secured loans, maturity period	3 years
Maximum
Schedule Of Significant Accounting Policies [Line Items]
Senior secured loans, maturity period	8 years
Interest Rate Swaps
Schedule Of Significant Accounting Policies [Line Items]
Centrally cleared interest rate swaps at fair value, reduction	$ 873,400,000
Cash on margin with counterparty to interest rate swaps	$ 884,700,000		$ 579,200,000