MORTGAGE LOANS - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Mortgage loans holdings	$ 7,701,465,000		$ 7,724,733,000
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	$ 77,700,000
Amount that would become due in 2020 through 2024, if loans are called at their next call dates	837,400,000
Amount that would become due in 2025 through 2029, if loans are called at their next call dates	60,600,000
Commercial mortgage loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Mortgage loans having participation feature	727,800,000		700,600,000
Income recognized on participating mortgage loans	2,200,000	$ 7,300,000
Loans not subject to a pooling and servicing agreement which are either nonperforming or restructured	2,700,000.0
Allowance for mortgage loan credit losses	$ 1,946,000		$ 1,296,000	$ 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
Past due period at which to initiate foreclosure proceedings (more than)	90 days
Commercial mortgage loans | Maximum
Financing Receivable, Allowance for Credit Losses [Line Items]
Loan-to-value ratio with participating interest (as a percent)	85.00%