FAIR VALUE OF FINANCIAL INSTRUMENTS - Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets:
Mortgage loans on real estate	$ 7,701,465	$ 7,724,733
Policy loans	1,677,442	1,695,886
Fixed maturities, held-to-maturity	2,607,356	2,633,474
Liabilities:
Stable value product account balances	5,527,816	5,234,731
Future policy benefits and claims	13,665,415	13,720,081
Other policyholders’ funds	1,166,378	1,128,379
Debt:
Subordinated debt	605,460	605,426
Non-recourse funding obligations	2,607,021	2,632,497
Capital lease obligation	2,800	0
Golden Gate V
Debt:
Non-recourse funding obligations	743,981	729,454
Level 3 | Carrying Amounts
Assets:
Mortgage loans on real estate	7,701,465	7,724,733
Policy loans	1,677,442	1,695,886
Fixed maturities, held-to-maturity	2,607,356	2,633,474
Liabilities:
Stable value product account balances	5,527,816	5,234,731
Future policy benefits and claims	1,637,741	1,671,414
Other policyholders’ funds	105,141	131,150
Debt:
Bank borrowings	0	0
Subordinated funding obligations	110,000	110,000
Non-recourse funding obligations	2,607,021	2,632,497
Level 3 | Carrying Amounts | Golden Gate V
Debt:
Non-recourse funding obligations	2,500,000	2,600,000
Level 3 | Fair Values
Assets:
Mortgage loans on real estate	7,624,622	7,447,702
Policy loans	1,677,442	1,695,886
Fixed maturities, held-to-maturity	2,594,441	2,547,210
Liabilities:
Stable value product account balances	5,536,721	5,200,723
Future policy benefits and claims	1,644,622	1,671,434
Other policyholders’ funds	107,975	131,782
Debt:
Bank borrowings	0	0
Subordinated funding obligations	99,026	95,476
Non-recourse funding obligations	2,656,379	2,550,237
Level 3 | Fair Values | Golden Gate V
Debt:
Non-recourse funding obligations	2,600,000	2,500,000
Level 2 | Carrying Amounts
Debt:
Senior Notes	1,080,878	1,100,508
Subordinated debt	495,460	495,426
Level 2 | Fair Values
Debt:
Senior Notes	1,076,623	1,065,338
Subordinated debt	$ 501,945	$ 494,265