MORTGAGE LOANS - Narrative (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) loan	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) restructuring loan category	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Financing Receivable, Allowance for Credit Losses [Line Items]
Mortgage loans holdings	$ 7,516,192,000		$ 7,516,192,000		$ 6,817,723,000
Mortgage loan call option period			12 years
Amount that would become due in remainder of 2018, if loans are called at their next call dates	114,200,000		$ 114,200,000
Amount that would become due in 2019 through 2023, if loans are called at their next call dates	916,600,000		916,600,000
Amount that would become due in 2024 through 2028, if loans are called at their next call dates	104,100,000		104,100,000
Amount that would become due after 2028, if loans are called at their next call dates	1,900,000		1,900,000
Loans not subject to a pooling and servicing agreement which are either nonperforming or restructured	$ 0		$ 0
Number of troubled debt restructurings | restructuring			0
Number of nonperforming loans which have been restructured | loan			0
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	$ 722,200,000		$ 722,200,000		669,300,000
Income recognized on participating mortgage loans	5,100,000	$ 4,700,000	12,400,000	$ 11,500,000
Loans not subject to a pooling and servicing agreement which are either nonperforming or restructured	0		$ 0
Number of loan categories | category			2
Loans not subject to a pooling and servicing agreement which are either nonperforming or restructured	0		$ 0
Allowance for mortgage loan credit losses	$ 970,000		$ 970,000		$ 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%