FAIR VALUE OF FINANCIAL INSTRUMENTS - Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets:
Mortgage loans on real estate	$ 7,572,349	$ 6,817,723
Policy loans	1,703,462	1,615,615
Fixed maturities, held-to-maturity	2,653,605	2,718,904
Liabilities:
Stable value product account balances	5,211,668	4,698,371
Future policy benefits and claims	13,590,482	10,921,190
Other policyholders’ funds	1,142,522	1,267,198
Debt:
Subordinated debt	605,392	495,289
Non-recourse funding obligations	2,651,967	2,747,477
Capital lease obligation	1,400	0
Golden Gate V
Debt:
Non-recourse funding obligations	714,921	681,285
Level 3 | Carrying Amounts
Assets:
Mortgage loans on real estate	7,572,349	6,817,723
Policy loans	1,703,462	1,615,615
Fixed maturities, held-to-maturity	2,653,605	2,718,904
Liabilities:
Stable value product account balances	5,211,668	4,698,371
Future policy benefits and claims	223,091	220,498
Other policyholders’ funds	131,338	133,508
Debt:
Bank borrowings	0	0
Subordinated funding obligations	110,000	0
Non-recourse funding obligations	2,651,967	2,747,477
Level 3 | Carrying Amounts | Golden Gate V
Debt:
Non-recourse funding obligations	2,600,000	2,700,000
Level 3 | Fair Values
Assets:
Mortgage loans on real estate	7,335,547	6,740,177
Policy loans	1,703,462	1,615,615
Fixed maturities, held-to-maturity	2,577,841	2,776,327
Liabilities:
Stable value product account balances	5,147,084	4,698,868
Future policy benefits and claims	223,091	220,498
Other policyholders’ funds	130,672	134,253
Debt:
Bank borrowings	0	0
Subordinated funding obligations	97,451	0
Non-recourse funding obligations	2,580,278	2,804,983
Level 3 | Fair Values | Golden Gate V
Debt:
Non-recourse funding obligations	2,600,000	2,800,000
Level 2 | Carrying Amounts
Debt:
Senior Notes	1,105,978	943,370
Subordinated debt	495,392	495,289
Level 2 | Fair Values
Debt:
Senior Notes	1,074,695	933,926
Subordinated debt	$ 497,760	$ 501,215