MORTGAGE LOANS - Narrative (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) restructuring	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Financing Receivable, Allowance for Credit Loss [Line Items]
Mortgage loans holdings	$ 9,327,911		$ 9,327,911		$ 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	54,400		$ 54,400
Amount that would become due in 2020 through 2024, if loans are called at their next call dates	850,900		850,900
Amount that would become due in 2025 through 2029, if loans are called at their next call dates	59,400		59,400
Commercial mortgage loans
Financing Receivable, Allowance for Credit Loss [Line Items]
Mortgage loans having participation feature	757,400		757,400		700,600
Income recognized on participating mortgage loans	3,800	$ 9,500	18,000	$ 22,000
Loans not subject to a pooling and servicing agreement which are either nonperforming or restructured	3,200		$ 3,200
Number of troubled debt restructurings | restructuring			4
Allowance for mortgage loan credit losses	$ 4,056		$ 4,056		$ 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%