FAIR VALUE OF FINANCIAL INSTRUMENTS - Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets:
Mortgage loans on real estate	$ 7,724,733	$ 6,817,723
Policy loans	1,695,886	1,615,615
Fixed maturities, held-to-maturity	2,633,474	2,718,904
Liabilities:
Stable value product account balances	5,234,731	4,698,371
Future policy benefits and claims	13,720,081	10,921,190
Other policyholders’ funds	1,128,379	1,267,198
Debt
Non-recourse funding obligations	2,888,329	2,952,822
Capital lease obligations	1,300	1,700
Golden Gate V
Debt
Non-recourse funding obligations	729,454	681,285
Carrying Amounts
Assets:
Mortgage loans on real estate	7,724,733	6,817,723
Policy loans	1,695,886	1,615,615
Fixed maturities, held-to-maturity	2,633,474	2,718,904
Liabilities:
Stable value product account balances	5,234,731	4,698,371
Future policy benefits and claims	1,671,414	220,498
Other policyholders’ funds	131,150	133,508
Debt
Non-recourse funding obligations	2,888,329	2,952,822
Carrying Amounts \| Golden Gate V
Debt
Non-recourse funding obligations	2,600,000	2,700,000
Carrying Amounts \| Level 3
Debt
Subordinated Debt Obligations, Fair Value Disclosure	110,000	0
Fair Value \| Golden Gate V
Debt
Non-recourse funding obligations	2,500,000
Fair Value \| Level 3
Assets:
Mortgage loans on real estate	7,447,702	6,740,177
Policy loans	1,695,886	1,615,615
Fixed maturities, held-to-maturity	2,547,210	2,776,327
Liabilities:
Stable value product account balances	5,200,723	4,698,868
Future policy benefits and claims	1,671,434	220,498
Other policyholders’ funds	131,782	134,253
Debt
Non-recourse funding obligations	2,801,399	2,980,495
Subordinated Debt Obligations, Fair Value Disclosure	$ 95,476	0
Fair Value \| Level 3 \| Golden Gate V
Debt
Non-recourse funding obligations		$ 2,800,000

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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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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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