DERIVATIVE FINANCIAL INSTRUMENTS - Nature and Accounting Treatment of Derivative Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount, Other long-term investments	$ 12,361,869	$ 11,437,051
Fair Value, Other long-term investments	629,537	533,433
Notional amount, Other liabilities	12,710,731	11,719,182
Fair value, liabilities	905,973	674,048
Foreign Currency Swaps | Cash flow hedges
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount, Other long-term investments	117,178	117,178
Fair Value, Other long-term investments	6,016	132
Interest rate swaps | Derivatives not designated as hedging instruments
Derivative Instruments, Gain (Loss) [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
Total Return Swap | Derivatives not designated as hedging instruments
Derivative Instruments, Gain (Loss) [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
Modco trading portfolio | Derivatives not designated as hedging instruments
Derivative Instruments, Gain (Loss) [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
Embedded derivative - GLWB | Derivatives not designated as hedging instruments
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount, Other long-term investments	2,116,935	2,045,529
Fair Value, Other long-term investments	67,879	64,064
Notional amount, Other liabilities	1,939,320	1,849,400
Fair value, liabilities	83,427	71,082
Interest rate futures | Derivatives not designated as hedging instruments
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount, Other long-term investments	1,071,870	102,587
Fair Value, Other long-term investments	3,178	894
Notional amount, Other liabilities	230,404	993,842
Fair value, liabilities	917	6,611
Equity futures | Derivatives not designated as hedging instruments
Derivative Instruments, Gain (Loss) [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
Currency futures | Derivatives not designated as hedging instruments
Derivative Instruments, Gain (Loss) [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
Equity options | Derivatives not designated as hedging instruments
Derivative Instruments, Gain (Loss) [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
Interest rate swaptions | Derivatives not designated as hedging instruments
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount, Other long-term investments	225,000	225,000
Fair Value, Other long-term investments	14	2,503
Other derivatives | Derivatives not designated as hedging instruments
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount, Other long-term investments	157	212
Fair Value, Other long-term investments	200	149
Funds withheld derivative | Derivatives not designated as hedging instruments
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount, Other liabilities	1,502,726	1,557,237
Fair value, liabilities	61,729	91,267
Embedded derivative - FIA | Derivatives not designated as hedging instruments
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount, Other liabilities	1,951,650	1,496,346
Fair value, liabilities	218,676	147,368
Embedded derivative - IUL | Derivatives not designated as hedging instruments
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount, Other liabilities	168,349	103,838
Fair value, liabilities	80,212	46,051
PLC | Derivatives not designated as hedging instruments
Derivative Instruments, Gain (Loss) [Line Items]
Notional amount, Other long-term investments	2,810,469	2,808,807
Fair Value, Other long-term investments	$ 91,578	$ 48,878