FAIR VALUE OF FINANCIAL INSTRUMENTS - Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Assets:
Mortgage loans on real estate	$ 6,528,890	$ 6,132,125
Policy loans	1,625,960	1,650,240
Fixed maturities, held-to-maturity, Carrying Amount	2,730,995	2,770,177
Fixed maturities, held-to-maturity, Fair Values	2,796,603	2,733,340
Liabilities:
Stable value product account balances	4,793,890	3,501,636
Annuity account balances	10,813,795	10,642,115
Other policyholders’ funds	1,265,500	1,165,749
Debt:
Subordinated debt securities	495,255	441,202
Non-recourse funding obligations	2,759,640	2,796,474
Capital lease obligation	1,500
Golden Gate V
Debt:
Non-recourse funding obligations	666,729	628,025
Fixed maturities
Assets:
Fixed maturities, held-to-maturity, Fair Values	2,796,603	2,733,340
Level 3 \| Carrying Amounts
Assets:
Mortgage loans on real estate	6,528,890	6,132,125
Policy loans	1,625,960	1,650,240
Liabilities:
Stable value product account balances	4,793,890	3,501,636
Annuity account balances	222,079	221,634
Other policyholders’ funds	132,565	135,367
Debt:
Bank borrowings	170,000	170,000
Non-recourse funding obligations	2,759,640	2,796,474
Level 3 \| Carrying Amounts \| Golden Gate V
Debt:
Non-recourse funding obligations	2,700,000	2,700,000
Level 3 \| Carrying Amounts \| Fixed maturities
Assets:
Fixed maturities, held-to-maturity, Carrying Amount	2,730,995	2,770,177
Level 3 \| Fair Values
Assets:
Mortgage loans on real estate	6,486,449	5,930,992
Policy loans	1,625,960	1,650,240
Liabilities:
Stable value product account balances	4,816,919	3,488,877
Annuity account balances	222,079	221,658
Other policyholders’ funds	133,347	136,127
Debt:
Bank borrowings	170,000	170,000
Non-recourse funding obligations	2,824,782	2,765,558
Level 3 \| Fair Values \| Golden Gate V
Debt:
Non-recourse funding obligations	2,700,000	2,700,000
Level 3 \| Fair Values \| Fixed maturities
Assets:
Fixed maturities, held-to-maturity, Fair Values	2,796,603	2,733,340
Level 2 \| Carrying Amounts
Debt:
Senior Notes	950,505	993,285
Subordinated debt securities	495,255	441,202
Level 2 \| Fair Values
Debt:
Senior Notes	936,497	937,074
Subordinated debt securities	$ 496,065	$ 443,355

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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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Fair value 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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