DERIVATIVE FINANCIAL INSTRUMENTS - Nature and Accounting Treatment of Derivative Financial Instruments (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Other long-term investments, Fair Value	$ 536,727	$ 605,075
Embedded derivative - Modco reinsurance treaties
Derivative [Line Items]
Other long-term investments, Fair Value	1,551	1,009
Embedded derivative - GLWB
Derivative [Line Items]
Other long-term investments, Fair Value	155,123	134,995
Derivatives designated as hedging instruments | Cash flow hedges | Foreign currency swaps
Derivative [Line Items]
Other long-term investments, Notional Amount	117,178	117,178
Other long-term investments, Fair Value	10,352	6,016
Derivatives not designated as hedging instruments
Derivative [Line Items]
Other long-term investments, Notional Amount	13,921,990	12,331,534
Other long-term investments, Fair Value	536,727	605,075
Other liabilities, Notional Amount	14,647,137	13,986,996
Other liabilities, Fair Value	759,264	1,007,572
Derivatives not designated as hedging instruments | Interest rate swaps
Derivative [Line Items]
Other long-term investments, Notional Amount	1,015,000	1,265,000
Other long-term investments, Fair Value	19,195	55,411
Other liabilities, Notional Amount	1,097,500	597,500
Other liabilities, Fair Value	18,538	2,960
Derivatives not designated as hedging instruments | Total return swaps
Derivative [Line Items]
Other long-term investments, Notional Amount	455,589	190,938
Other long-term investments, Fair Value	12,419	135
Other liabilities, Notional Amount	0	243,388
Other liabilities, Fair Value	0	318
Derivatives not designated as hedging instruments | Embedded derivative - Modco reinsurance treaties
Derivative [Line Items]
Other long-term investments, Notional Amount	326,046	64,472
Other long-term investments, Fair Value	1,551	1,009
Other liabilities, Notional Amount	2,097,197	2,390,539
Other liabilities, Fair Value	76,145	215,247
Derivatives not designated as hedging instruments | Embedded derivative - GLWB
Derivative [Line Items]
Other long-term investments, Notional Amount	5,618,267	4,897,069
Other long-term investments, Fair Value	155,123	134,995
Other liabilities, Notional Amount	4,071,897	4,718,311
Other liabilities, Fair Value	175,036	246,755
Derivatives not designated as hedging instruments | Embedded derivative - FIA
Derivative [Line Items]
Other liabilities, Notional Amount	2,296,052	1,951,650
Other liabilities, Fair Value	238,569	218,676
Derivatives not designated as hedging instruments | Embedded derivative - IUL
Derivative [Line Items]
Other liabilities, Notional Amount	203,079	168,349
Other liabilities, Fair Value	83,166	80,212
Derivatives not designated as hedging instruments | Interest rate futures
Derivative [Line Items]
Other long-term investments, Notional Amount	352,736	1,071,870
Other long-term investments, Fair Value	6,037	3,178
Other liabilities, Notional Amount	914,621	230,404
Other liabilities, Fair Value	3,907	917
Derivatives not designated as hedging instruments | Equity futures
Derivative [Line Items]
Other long-term investments, Notional Amount	273,881	62,266
Other long-term investments, Fair Value	5,054	154
Other liabilities, Notional Amount	58,756	318,795
Other liabilities, Fair Value	1,199	2,593
Derivatives not designated as hedging instruments | Currency futures
Derivative [Line Items]
Other long-term investments, Notional Amount	204,542	1,117
Other long-term investments, Fair Value	2,680	2
Other liabilities, Notional Amount	0	255,248
Other liabilities, Fair Value	0	2,087
Derivatives not designated as hedging instruments | Equity options
Derivative [Line Items]
Other long-term investments, Notional Amount	5,558,594	4,436,467
Other long-term investments, Fair Value	324,123	403,961
Other liabilities, Notional Amount	3,908,035	3,112,812
Other liabilities, Fair Value	162,704	237,807
Derivatives not designated as hedging instruments | Interest rate swaptions
Derivative [Line Items]
Other long-term investments, Notional Amount	0	225,000
Other long-term investments, Fair Value	0	14
Derivatives not designated as hedging instruments | Other
Derivative [Line Items]
Other long-term investments, Notional Amount	157	157
Other long-term investments, Fair Value	$ 193	$ 200