SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Schedule Of Significant Accounting Policies [Line Items]
Real estate acquired in settlement of residential mortgage loans	$ 0		$ 0		$ 0
Other-than-temporary impairment recognized	0	$ 0	0	$ 0
Outstanding impaired loans, nonaccrual loans, and loans in default	0		0		0
Allowance for loan losses			$ 0		0
Minimum
Schedule Of Significant Accounting Policies [Line Items]
Senior secured loans, maturity period			2 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	1,200,000,000		$ 1,200,000,000
Cash on margin with counterparty to interest rate swaps	$ 1,000,000,000		$ 1,000,000,000		$ 579,200,000