MORTGAGE LOANS - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Loss [Line Items]
Mortgage loans holdings	$ 9,053,400		$ 9,053,400		$ 7,724,733
Mortgage loan call option period			10 years
Amount that would become due in remainder of 2019, if loans are called at their next call dates	70,300		$ 70,300
Amount that would become due in 2020 through 2024, if loans are called at their next call dates	789,600		789,600
Amount that would become due in 2025 through 2029, if loans are called at their next call dates	60,000		60,000
Commercial mortgage loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Mortgage loans having participation feature	669,500		669,500		700,600
Income recognized on participating mortgage loans	12,000	$ 5,100	14,200	$ 12,400
Loans not subject to a pooling and servicing agreement which are either nonperforming or restructured	800		800
Allowance for mortgage loan credit losses	$ 2,481		$ 2,481		$ 1,296	$ 0
Past due period at which to cease carrying accrued interest on delinquent loans			90 days
Number of days accrued interest on impaired loans (less than)			90 days
Commercial mortgage loans | Maximum
Financing Receivable, Allowance for Credit Loss [Line Items]
Loan-to-value ratio with participating interest (as a percent)	85.00%		85.00%