FAIR VALUE OF FINANCIAL INSTRUMENTS - Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Carrying amounts and estimated fair values of the Company's financial instruments
Capital Lease Obligations	$ 1,700
Assets:
Mortgage loans on real estate	6,817,723	$ 6,132,125
Policy loans	1,615,615	1,650,240
Fixed maturities, held-to-maturity	2,718,904	2,770,177
Liabilities:
Stable value product account balances	4,698,371	3,501,636
Annuity account balances	10,921,190	10,642,115
Other policyholders' funds	1,267,198	1,165,749
Debt:
Subordinated debt securities	495,289	441,202
Non-recourse funding obligations	2,747,477	2,796,474
Golden Gate V
Debt:
Non-recourse funding obligations	681,285	628,025
Carrying Amounts
Assets:
Mortgage loans on real estate	6,817,723	6,132,125
Policy loans	1,615,615	1,650,240
Fixed maturities, held-to-maturity	2,718,904	2,770,177
Liabilities:
Stable value product account balances	4,698,371	3,501,636
Annuity account balances	220,498	221,634
Other policyholders' funds	133,508	135,367
Debt:
Bank borrowings	0	170,000
Senior Notes	943,370	993,285
Subordinated debt securities	495,289	441,202
Non-recourse funding obligations	2,747,477	2,796,474
Carrying Amounts \| Golden Gate V
Debt:
Non-recourse funding obligations	2,700,000	2,700,000
Fair Values \| Golden Gate V
Debt:
Non-recourse funding obligations	2,800,000	2,700,000
Fair Values \| Level 3
Assets:
Mortgage loans on real estate	6,740,177	5,930,992
Policy loans	1,615,615	1,650,240
Fixed maturities, held-to-maturity	2,776,327	2,733,340
Liabilities:
Stable value product account balances	4,698,868	3,488,877
Annuity account balances	220,498	221,658
Other policyholders' funds	134,253	136,127
Debt:
Bank borrowings	0	170,000
Non-recourse funding obligations	2,804,983	2,765,558
Fair Values \| Level 2
Debt:
Senior Notes	933,926	937,074
Subordinated debt securities	$ 501,215	$ 443,355

Represents the liability as of the balance sheet date of amounts held for annuity contracts.

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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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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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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as 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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Including the current and noncurrent portions, carrying value as of the balance sheet date of loans from a bank with maturities initially due after one year or beyond the normal operating cycle if longer.

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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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Including the current and noncurrent portions, carrying value as of the balance sheet date of subordinated debt (with initial maturities beyond one year or beyond the operating cycle if longer). Subordinated debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets.

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