FAIR VALUE OF FINANCIAL INSTRUMENTS - Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Assets:
Mortgage loans on real estate	$ 6,846,633	$ 6,817,723
Policy loans	1,594,642	1,615,615
Fixed maturities, held-to-maturity	2,699,826	2,718,904
Liabilities:
Stable value product account balances	4,699,614	4,698,371
Annuity account balances	11,060,885	10,921,190
Other policyholders’ funds	1,114,823	1,267,198
Debt:
Subordinated debt securities	495,324	495,289
Non-recourse funding obligations	2,728,689	2,747,477
Capital lease obligation	1,600
Golden Gate V
Debt:
Non-recourse funding obligations	695,836	681,285
Carrying Amounts
Assets:
Fixed maturities, held-to-maturity	2,699,826	2,718,904
Level 3 \| Carrying Amounts
Assets:
Mortgage loans on real estate	6,846,633	6,817,723
Policy loans	1,594,642	1,615,615
Liabilities:
Stable value product account balances	4,699,614	4,698,371
Annuity account balances	220,307	220,498
Other policyholders’ funds	135,202	133,508
Debt:
Bank borrowings	325,000	0
Non-recourse funding obligations	2,728,689	2,747,477
Level 3 \| Carrying Amounts \| Golden Gate V
Debt:
Non-recourse funding obligations	2,600,000	2,700,000
Level 3 \| Fair Values
Assets:
Mortgage loans on real estate	6,646,782	6,740,177
Policy loans	1,594,642	1,615,615
Fixed maturities, held-to-maturity	2,674,129	2,776,327
Liabilities:
Stable value product account balances	4,658,178	4,698,868
Annuity account balances	220,307	220,498
Other policyholders’ funds	135,921	134,253
Debt:
Bank borrowings	325,000	0
Non-recourse funding obligations	2,706,594	2,804,983
Level 3 \| Fair Values \| Golden Gate V
Debt:
Non-recourse funding obligations	2,700,000	2,800,000
Level 2 \| Carrying Amounts
Debt:
Senior Notes	769,776	943,370
Subordinated debt securities	495,324	495,289
Level 2 \| Fair Values
Debt:
Senior Notes	751,951	933,926
Subordinated debt securities	$ 486,595	$ 501,215

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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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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 Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer). Senior note holders are paid off in full before any payments are made to junior note holders.

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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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