FAIR VALUE OF FINANCIAL INSTRUMENTS - Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Assets:
Mortgage loans on real estate	$ 9,053,400	$ 7,724,733
Policy loans	1,709,110	1,695,886
Fixed maturities, held-to-maturity	2,576,216	2,633,474
Liabilities:
Stable value product account balances	5,816,503	5,234,731
Future policy benefits and claims	14,179,277	13,720,081
Other policyholders’ funds	1,542,296	1,128,379
Debt:
Subordinated debt	605,494	605,426
Non-recourse funding obligations	2,576,538	2,632,497
Capital lease obligation	2,600	1,300
Golden Gate V
Debt:
Non-recourse funding obligations	753,503	729,454
Level 3 | Carrying Amounts
Assets:
Mortgage loans on real estate	9,053,400	7,724,733
Policy loans	1,709,110	1,695,886
Fixed maturities, held-to-maturity	2,576,216	2,633,474
Liabilities:
Stable value product account balances	5,816,503	5,234,731
Future policy benefits and claims	1,725,194	1,671,414
Other policyholders’ funds	105,136	131,150
Debt:
Bank borrowings(5)	599,627	0
Subordinated funding obligations	110,000	110,000
Non-recourse funding obligations	2,576,538	2,632,497
Level 3 | Carrying Amounts | Golden Gate V
Debt:
Non-recourse funding obligations	2,600,000	2,600,000
Level 3 | Fair Values
Assets:
Mortgage loans on real estate	9,123,182	7,447,702
Policy loans	1,709,110	1,695,886
Fixed maturities, held-to-maturity	2,640,827	2,547,210
Liabilities:
Stable value product account balances	5,885,528	5,200,723
Future policy benefits and claims	1,729,081	1,671,434
Other policyholders’ funds	107,702	131,782
Debt:
Bank borrowings(5)	600,000	0
Subordinated funding obligations	108,455	95,476
Non-recourse funding obligations	2,701,194	2,550,237
Level 3 | Fair Values | Golden Gate V
Debt:
Non-recourse funding obligations	2,700,000	2,500,000
Level 2 | Carrying Amounts
Debt:
Senior Notes	1,075,370	1,100,508
Subordinated debt	495,494	495,426
Level 2 | Fair Values
Debt:
Senior Notes	1,095,907	1,065,338
Subordinated debt	$ 506,720	$ 494,265