MORTGAGE LOANS - Additional Information (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) loan	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) restructuring loan category	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Mortgage loans
Mortgage loans on real estate	$ 7,572,349,000		$ 7,572,349,000		$ 6,817,723,000
Period for exercise of call options or interest rate reset options			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
Financing receivable not subject to pooling and servicing agreement, non performing or restructured amount	$ 0		$ 0
Loans subject to a pooling and servicing agreement which are either nonperforming or restructured | loan	0		0
Number of nonperforming financing receivable subject to pooling and servicing agreement which have been restructured | loan			0
Commercial mortgage loans
Mortgage loans
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
Nonperforming mortgage loans, foreclosed properties and restructured loans to invested assets	3,000,000		$ 3,000,000
Number of loan categories | category			2
Financing receivable not subject to pooling and servicing agreement, non performing or restructured amount	3,000,000		$ 3,000,000
Allowance for mortgage loan credit losses	$ 1,481,000		$ 1,481,000		$ 0
Minimum number of days for ceasing accrued interest			90 days
Minimum number of days past due to initiate foreclosure proceedings			90 days
Maximum number of days accrued interest on impaired loans			90 days
Maximum | Commercial mortgage loans
Mortgage loans
Loan-to-value ratio with participating interest (as a percent)	85.00%		85.00%