DERIVATIVE FINANCIAL INSTRUMENTS - Nature and Accounting Treatment of Derivative Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional Amount, Other long-term investments	$ 12,331,534	$ 11,184,348
Fair Value, Other long-term investments	605,075	542,243
Notional Amount, Other liabilities	13,986,996	14,274,589
Fair value, liabilities	1,007,572	748,821
Embedded derivative - Modco reinsurance treaties
Derivative [Line Items]
Fair Value, Other long-term investments	1,009	2,573
Fair value, liabilities	215,247	141,301
Embedded derivative - GLWB
Derivative [Line Items]
Fair Value, Other long-term investments	134,995	121,752
Fair value, liabilities	246,755	237,122
Embedded derivative—FIA
Derivative [Line Items]
Fair value, liabilities	218,676	147,368
Embedded derivative—IUL
Derivative [Line Items]
Fair value, liabilities	80,212	46,051
Cash flow hedges | Foreign currency swaps
Derivative [Line Items]
Notional Amount, Other long-term investments	117,178	117,178
Fair Value, Other long-term investments	6,016	132
Derivatives not designated as hedging instruments | Interest rate swaps
Derivative [Line Items]
Notional Amount, Other long-term investments	1,265,000	1,135,000
Fair Value, Other long-term investments	55,411	71,644
Notional Amount, Other liabilities	597,500	575,000
Fair value, liabilities	2,960	10,208
Derivatives not designated as hedging instruments | Total Return Swap
Derivative [Line Items]
Notional Amount, Other long-term investments	190,938	0
Fair Value, Other long-term investments	135	0
Notional Amount, Other liabilities	243,388	0
Fair value, liabilities	318	0
Derivatives not designated as hedging instruments | Embedded derivative - Modco reinsurance treaties
Derivative [Line Items]
Notional Amount, Other long-term investments	64,472	64,123
Fair Value, Other long-term investments	1,009	2,573
Notional Amount, Other liabilities	2,390,539	2,450,692
Fair value, liabilities	215,247	141,301
Derivatives not designated as hedging instruments | Embedded derivative - GLWB
Derivative [Line Items]
Notional Amount, Other long-term investments	4,897,069	4,601,633
Fair Value, Other long-term investments	134,995	121,752
Notional Amount, Other liabilities	4,718,311	5,962,044
Fair value, liabilities	246,755	237,122
Derivatives not designated as hedging instruments | Interest rate futures
Derivative [Line Items]
Notional Amount, Other long-term investments	1,071,870	102,587
Fair Value, Other long-term investments	3,178	894
Derivatives not designated as hedging instruments | Equity futures
Derivative [Line Items]
Notional Amount, Other long-term investments	62,266	654,113
Fair Value, Other long-term investments	154	5,805
Notional Amount, Other liabilities	318,795	102,667
Fair value, liabilities	2,593	2,907
Derivatives not designated as hedging instruments | Currency futures
Derivative [Line Items]
Notional Amount, Other long-term investments	1,117	340,058
Fair Value, Other long-term investments	2	7,883
Notional Amount, Other liabilities	255,248	0
Fair value, liabilities	2,087	0
Derivatives not designated as hedging instruments | Equity options
Derivative [Line Items]
Notional Amount, Other long-term investments	4,436,467	3,944,444
Fair Value, Other long-term investments	403,961	328,908
Notional Amount, Other liabilities	3,112,812	2,590,160
Fair value, liabilities	237,807	157,253
Derivatives not designated as hedging instruments | Interest rate swaptions
Derivative [Line Items]
Notional Amount, Other long-term investments	225,000	225,000
Fair Value, Other long-term investments	14	2,503
Derivatives not designated as hedging instruments | Other derivatives
Derivative [Line Items]
Notional Amount, Other long-term investments	157	212
Fair Value, Other long-term investments	200	149
Derivatives not designated as hedging instruments | Embedded derivative—FIA
Derivative [Line Items]
Notional Amount, Other liabilities	1,951,650	1,496,346
Fair value, liabilities	218,676	147,368
Derivatives not designated as hedging instruments | Embedded derivative—IUL
Derivative [Line Items]
Notional Amount, Other liabilities	168,349	103,838
Fair value, liabilities	80,212	46,051
Derivatives not designated as hedging instruments | Interest rate futures
Derivative [Line Items]
Notional Amount, Other liabilities	230,404	993,842
Fair value, liabilities	$ 917	$ 6,611