SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Universal Life and Investment Products (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 31, 2015
Universal Life | Minimum
CERTAIN NONTRADITIONAL LONG-DURATION CONTRACTS
Interest rate credited to policy account balances	1.00%
Universal Life | Maximum
CERTAIN NONTRADITIONAL LONG-DURATION CONTRACTS
Interest rate credited to policy account balances	8.75%
Investments Product Line | Minimum
CERTAIN NONTRADITIONAL LONG-DURATION CONTRACTS
Interest rate credited to policy account balances	0.50%
Investments Product Line | Maximum
CERTAIN NONTRADITIONAL LONG-DURATION CONTRACTS
Interest rate credited to policy account balances	11.30%
Annuity account | Guaranteed minimum death benefits (GMDB)
CERTAIN NONTRADITIONAL LONG-DURATION CONTRACTS
Guaranteed benefit liability	$ 34,030	$ 34,796	$ 36,427	$ 29,010