FAIR VALUE OF FINANCIAL INSTRUMENTS - Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets:
Mortgage loans on real estate	$ 7,516,192	$ 6,817,723
Policy loans	1,721,743	1,615,615
Fixed maturities, held-to-maturity	2,677,726	2,718,904
Liabilities:
Stable value product account balances	5,025,930	4,698,371
Future policy benefits and claims	13,303,399	10,921,190
Other policyholders’ funds	1,161,767	1,267,198
Debt
Non-recourse funding obligations	2,913,621	2,952,822
Capital lease obligations	1,600
Level 3 | Carrying Amounts
Assets:
Mortgage loans on real estate	7,516,192	6,817,723
Policy loans	1,721,743	1,615,615
Liabilities:
Stable value product account balances	5,025,930	4,698,371
Future policy benefits and claims	224,249	220,498
Other policyholders’ funds	134,976	133,508
Debt
Non-recourse funding obligations	2,913,621	2,952,822
Subordinated funding obligations	110,000	0
Level 3 | Carrying Amounts | Golden Gate and 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	2,677,726	2,718,904
Level 3 | Fair Values
Assets:
Mortgage loans on real estate	7,330,137	6,740,177
Policy loans	1,721,743	1,615,615
Liabilities:
Stable value product account balances	4,978,498	4,698,868
Future policy benefits and claims	244,249	220,498
Other policyholders’ funds	135,631	134,253
Debt
Non-recourse funding obligations	2,820,229	2,980,495
Subordinated funding obligations	98,775	0
Level 3 | Fair Values | Golden Gate and 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	$ 2,582,938	$ 2,776,327