MORTGAGE LOANS - Additional Information (Details)	3 Months Ended
	Mar. 31, 2018 USD ($) category loan	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Mortgage loans
Mortgage loans on real estate	$ 6,846,633,000		$ 6,817,723,000
Period for exercise of call options or interest rate reset options	12 years
Amount that would become due in remainder of 2018, if loans are called at their next call dates	$ 143,600,000
Amount that would become due in 2019 through 2023, if loans are called at their next call dates	873,100,000
Amount that would become due in 2024 through 2028, if loans are called at their next call dates	105,000,000
Amount that would become due after 2028, if loans are called at their next call dates	2,000,000
Commercial mortgage loans
Mortgage loans
Mortgage loans having participation feature	672,100,000		669,300,000
Income recognized on participating mortgage loans	7,300,000	$ 6,800,000
Nonperforming mortgage loans, foreclosed properties and restructured loans to invested assets	$ 0
Number of loan categories | category	2
Financing receivable not subject to pooling and servicing agreement, non performing or restructured amount	$ 0
Loans foreclosed not subject to a pooling and servicing agreement	$ 0
Loans subject to a pooling and servicing agreement which are either nonperforming or restructured | loan	0
Loans foreclosed subject to a pooling and servicing agreement	$ 0
Allowance for mortgage loan credit losses	$ 0		$ 0
Maximum number of days accrued interest on impaired loans	90 days
Minimum number of days past due to initiate foreclosure proceedings	90 days
Maximum | Commercial mortgage loans
Mortgage loans
Loan-to-value ratio with participating interest (as a percent)	85.00%