DERIVATIVE FINANCIAL INSTRUMENTS - Nature and Accounting Treatment of Derivative Instruments (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Derivative asset, notional amount	$ 12,146,013	$ 12,331,534
Derivative asset, fair value	375,829	605,075
Derivative liability, notional amount	21,010,029	13,986,996
Derivative liability, fair value	755,461	1,007,572
Embedded derivative - Modco reinsurance treaties
Derivative [Line Items]
Derivative asset, fair value	7,072	1,009
Derivative liability, fair value	32,828	215,247
Embedded derivative - GLWB
Derivative [Line Items]
Derivative asset, fair value	105,272	134,995
Derivative liability, fair value	289,343	246,755
Embedded derivative—FIA
Derivative [Line Items]
Derivative liability, fair value	217,288	218,676
Embedded derivative—IUL
Derivative [Line Items]
Derivative liability, fair value	90,231	80,212
Cash flow hedges | Foreign currency swaps
Derivative [Line Items]
Derivative asset, notional amount	0	117,178
Derivative asset, fair value	0	6,016
Derivatives not designated as hedging instruments | Interest rate swaps
Derivative [Line Items]
Derivative asset, notional amount	1,515,500	1,265,000
Derivative asset, fair value	28,501	55,411
Derivative liability, notional amount	775,000	597,500
Derivative liability, fair value	11,367	2,960
Derivatives not designated as hedging instruments | Total return swaps
Derivative [Line Items]
Derivative asset, notional amount	138,070	190,938
Derivative asset, fair value	3,971	135
Derivative liability, notional amount	768,177	243,388
Derivative liability, fair value	23,054	318
Derivatives not designated as hedging instruments | Embedded derivative - Modco reinsurance treaties
Derivative [Line Items]
Derivative asset, notional amount	585,294	64,472
Derivative asset, fair value	7,072	1,009
Derivative liability, notional amount	1,795,287	2,390,539
Derivative liability, fair value	32,828	215,247
Derivatives not designated as hedging instruments | Embedded derivative - GLWB
Derivative [Line Items]
Derivative asset, notional amount	3,984,070	4,897,069
Derivative asset, fair value	105,272	134,995
Derivative liability, notional amount	8,466,019	4,718,311
Derivative liability, fair value	289,343	246,755
Derivatives not designated as hedging instruments | Embedded derivative—FIA
Derivative [Line Items]
Derivative liability, notional amount	2,576,033	1,951,650
Derivative liability, fair value	217,288	218,676
Derivatives not designated as hedging instruments | Embedded derivative—IUL
Derivative [Line Items]
Derivative liability, notional amount	233,550	168,349
Derivative liability, fair value	90,231	80,212
Derivatives not designated as hedging instruments | Interest rate futures
Derivative [Line Items]
Derivative asset, notional amount	286,208	1,071,870
Derivative asset, fair value	10,302	3,178
Derivative liability, notional amount	863,706	230,404
Derivative liability, fair value	20,100	917
Derivatives not designated as hedging instruments | Equity futures
Derivative [Line Items]
Derivative asset, notional amount	12,633	62,266
Derivative asset, fair value	483	154
Derivative liability, notional amount	659,357	318,795
Derivative liability, fair value	33,753	2,593
Derivatives not designated as hedging instruments | Currency futures
Derivative [Line Items]
Derivative asset, notional amount	0	1,117
Derivative asset, fair value	0	2
Derivative liability, notional amount	202,747	255,248
Derivative liability, fair value	2,163	2,087
Derivatives not designated as hedging instruments | Equity options
Derivative [Line Items]
Derivative asset, notional amount	5,624,081	4,436,467
Derivative asset, fair value	220,092	403,961
Derivative liability, notional amount	4,199,687	3,112,812
Derivative liability, fair value	34,178	237,807
Derivatives not designated as hedging instruments | Interest rate swaptions
Derivative [Line Items]
Derivative asset, notional amount	0	225,000
Derivative asset, fair value	0	14
Derivatives not designated as hedging instruments | Other derivatives
Derivative [Line Items]
Derivative asset, notional amount	157	157
Derivative asset, fair value	136	200
Derivative liability, notional amount	3,288	0
Derivative liability, fair value	252	0
Cash flow hedges | Cash flow hedges | Foreign currency swaps
Derivative [Line Items]
Derivative liability, notional amount	117,178	0
Derivative liability, fair value	904	0
Cash flow hedges | Cash flow hedges | Interest rate swaps
Derivative [Line Items]
Derivative liability, notional amount	350,000	0
Derivative liability, fair value	$ 0	$ 0