FAIR VALUE OF FINANCIAL INSTRUMENTS - Estimated Fair Value of Financial Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets:
Commercial mortgage loans	$ 10,005,562	$ 9,379,401
Policy loans	1,593,394	1,675,121
Fixed maturities, held-to-maturity		2,823,881
Other long-term investments	3,164,180	2,479,520
Liabilities:
Future policy benefits and claims	15,477,640	14,289,907
Other policyholders’ funds	1,865,421	1,576,856
Debt
Non-recourse funding obligations	2,197	3,082,753
Capital lease obligations	600	1,000
Level 3
Liabilities:
Stable value product account balances	6,056,181	5,443,752
Carrying Amounts | Golden Gate and Golden Gate V
Debt
Non-recourse funding obligations		2,800,000
Carrying Amounts | Level 3
Assets:
Commercial mortgage loans	10,005,562	9,379,401
Policy loans	1,593,394	1,675,121
Fixed maturities, held-to-maturity	0	2,823,881
Other long-term investments	1,186,063	1,216,996
Liabilities:
Future policy benefits and claims	1,580,221	1,701,324
Other policyholders’ funds	102,091	127,084
Debt
Non-recourse funding obligations	2,197	3,082,753
Subordinated funding obligations	110,000	110,000
Fair Value | Level 3
Assets:
Commercial mortgage loans	10,787,782	9,584,487
Policy loans	1,593,394	1,675,121
Fixed maturities, held-to-maturity	0	3,025,790
Other long-term investments	1,282,999	1,246,889
Liabilities:
Stable value product account balances	6,230,663	5,551,195
Future policy benefits and claims	1,602,813	1,705,235
Other policyholders’ funds	107,864	130,259
Debt
Non-recourse funding obligations	2,486	3,298,580
Subordinated funding obligations	$ 120,835	113,286
Fair Value | Level 3 | Golden Gate and Golden Gate V
Debt
Non-recourse funding obligations		$ 3,000,000