FAIR VALUE OF FINANCIAL INSTRUMENTS - Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets:
Mortgage loans on real estate	$ 7,572,349	$ 6,817,723
Policy loans	1,703,462	1,615,615
Fixed maturities, held-to-maturity	2,653,605	2,718,904
Liabilities:
Stable value product account balances	5,211,668	4,698,371
Future policy benefits and claims	13,590,482	10,921,190
Other policyholders’ funds	1,142,522	1,267,198
Debt
Non-recourse funding obligations	2,890,435	2,952,822
Capital lease obligations	1,400	1,700
Golden Gate V
Debt
Non-recourse funding obligations	714,921	681,285
Level 3 \| Carrying Amounts
Assets:
Mortgage loans on real estate	7,572,349	6,817,723
Policy loans	1,703,462	1,615,615
Liabilities:
Stable value product account balances	5,211,668	4,698,371
Future policy benefits and claims	223,091	220,498
Other policyholders’ funds	131,338	133,508
Debt
Non-recourse funding obligations	2,890,435	2,952,822
Subordinated funding obligations	110,000	0
Level 3 \| Carrying Amounts \| Golden Gate V
Debt
Non-recourse funding obligations	2,600,000	2,700,000
Level 3 \| Carrying Amounts \| Fixed maturities
Assets:
Fixed maturities, held-to-maturity	2,653,605	2,718,904
Level 3 \| Fair Values
Assets:
Mortgage loans on real estate	7,335,547	6,740,177
Policy loans	1,703,462	1,615,615
Liabilities:
Stable value product account balances	5,147,084	4,698,868
Future policy benefits and claims	223,091	220,498
Other policyholders’ funds	130,672	134,253
Debt
Non-recourse funding obligations	2,814,355	2,980,495
Subordinated funding obligations	97,451	0
Level 3 \| Fair Values \| Golden Gate V
Debt
Non-recourse funding obligations	2,600,000	2,800,000
Level 3 \| Fair Values \| Fixed maturities
Assets:
Fixed maturities, held-to-maturity	$ 2,577,841	$ 2,776,327

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Non-recourse funding obligations issued by special purpose entities owned by the reporting entity. The funding obligations are direct financial obligations of the special purpose entity and are not guaranteed by the reporting entity.

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Represents the liability as of the balance sheet date of amounts related to stable value contracts.

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Amount equal to the present value (the principal) at the beginning of the lease term of minimum lease payments during the lease term (excluding that portion of the payments representing executory costs such as insurance, maintenance, and taxes to be paid by the lessor, together with any profit thereon) net of payments or other amounts applied to the principal through the balance sheet date.

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Fair value of subordinated debt obligations including, but not limited to, subordinated loan, subordinated bond, subordinated debenture or junior debt.

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Amount of investment in debt security measured at amortized cost (held-to-maturity).

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Amount before allowance of loans made to policy and contract holders using their life insurance contracts' cash value or separate accounts as collateral. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums over and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

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The balance represents the amount of loans that are secured by real estate mortgages, offset by the reserve to cover probable credit losses on the loan portfolio.

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Amount due to policyholders for funds held that are returnable under the terms of insurance contracts, classified as other.

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