MORTGAGE LOANS - Narrative (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) loan	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) restructuring category loan	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Financing Receivable, Allowance for Credit Losses [Line Items]
Mortgage loans holdings	$ 7,572,349,000		$ 7,572,349,000		$ 6,817,723,000
Mortgage loan call option period			11 years
Amount that would become due in remainder of 2018, if loans are called at their next call dates	80,700,000		$ 80,700,000
Amount that would become due in 2019 through 2023, if loans are called at their next call dates	933,400,000		933,400,000
Amount that would become due in 2024 through 2028, if loans are called at their next call dates	91,800,000		91,800,000
Amount that would become due after 2028, if loans are called at their next call dates	$ 1,900,000		$ 1,900,000
Number of troubled debt restructurings | restructuring			1
Loans subject to pooling and servicing agreement, which are either nonperforming or restructured | loan	0		0
Commercial mortgage loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Mortgage loans having participation feature	$ 757,000,000		$ 757,000,000		669,300,000
Income recognized on participating mortgage loans	9,500,000	$ 14,200,000	22,000,000	$ 25,700,000
Loans not subject to a pooling and servicing agreement which are either nonperforming or restructured	3,000,000.0		$ 3,000,000.0
Number of loan categories | category			2
Loans not subject to a pooling and servicing agreement which are either nonperforming or restructured	3,000,000.0		$ 3,000,000.0
Allowance for mortgage loan credit losses	$ 1,481,000		$ 1,481,000		$ 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%		85.00%