FAIR VALUE OF FINANCIAL INSTRUMENTS - Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Assets:
Mortgage loans on real estate	$ 9,327,911	$ 7,724,733
Policy loans	1,686,832	1,695,886
Fixed maturities, held-to-maturity	2,544,054	2,633,474
Liabilities:
Stable value product account balances	5,450,981	5,234,731
Future policy benefits and claims	14,181,458	13,720,081
Other policyholders’ funds	1,622,475	1,128,379
Debt:
Subordinated debt	605,528	605,426
Non-recourse funding obligations	2,545,049	2,632,497
Capital lease obligation	2,300	1,300
Golden Gate V
Debt:
Non-recourse funding obligations	763,018	729,454
Level 3 | Carrying Amounts
Assets:
Mortgage loans on real estate	9,327,911	7,724,733
Policy loans	1,686,832	1,695,886
Fixed maturities, held-to-maturity	2,544,054	2,633,474
Liabilities:
Stable value product account balances	5,450,981	5,234,731
Future policy benefits and claims	1,707,238	1,671,414
Other policyholders’ funds	104,395	131,150
Debt:
Bank borrowings(5)	599,653	0
Subordinated funding obligations	110,000	110,000
Non-recourse funding obligations	2,545,049	2,632,497
Level 3 | Carrying Amounts | Golden Gate V
Debt:
Non-recourse funding obligations	2,700,000	2,600,000
Level 3 | Fair Values
Assets:
Mortgage loans on real estate	9,529,865	7,447,702
Policy loans	1,686,832	1,695,886
Fixed maturities, held-to-maturity	2,685,076	2,547,210
Liabilities:
Stable value product account balances	5,547,509	5,200,723
Future policy benefits and claims	1,709,444	1,671,434
Other policyholders’ funds	106,285	131,782
Debt:
Bank borrowings(5)	600,000	0
Subordinated funding obligations	113,369	95,476
Non-recourse funding obligations	2,723,594	2,550,237
Level 3 | Fair Values | Golden Gate V
Debt:
Non-recourse funding obligations	2,500,000	2,500,000
Level 2 | Carrying Amounts
Debt:
Senior Notes	1,467,245	1,100,508
Subordinated debt	495,528	495,426
Level 2 | Fair Values
Debt:
Senior Notes	1,531,330	1,065,338
Subordinated debt	$ 511,690	$ 494,265