FAIR VALUE OF FINANCIAL INSTRUMENTS - Reconciliation of Fair Value Measurements for Level 3 (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Transfers
Securities transferred into Level 3	$ 184,000,000.0	$ 195,400,000
Transferred out of Level 3	1,200,000	58,400,000
Transfers from Level 2 to Level 1		0
Level 3
Assets:
Beginning Balance	2,164,553,000	1,306,979,000
Total Realized and Unrealized Gains Included in Net Income	346,497,000	98,769,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	144,598,000	99,401,000
Total Realized and Unrealized Losses Included in Net Income	(237,843,000)	(35,267,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(97,398,000)	(22,993,000)
Purchases	525,379,000	807,316,000
Sales	(708,489,000)	(223,139,000)
Issuances	0	0
Settlements	(4,072,000)	(1,708,000)
Transfers in/out of Level 3	182,821,000	137,093,000
Other	(4,745,000)	(1,898,000)
Ending Balance	2,311,301,000	2,164,553,000
Total Gains (losses) included in Net Income related to Instruments still held at the  Reporting Date	88,643,000	59,212,000
Liabilities:
Beginning Balance	1,087,700,000	514,246,000
Total Realized and Unrealized Gains Included in Net Income	502,098,000	108,438,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Net Income	(1,225,278,000)	(619,945,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	0	70,888,000
Sales	0	0
Issuances	433,000	0
Settlements	5,863,000	365,000
Transfers in/out of Level 3	0	9,306,000
Other	0	0
Ending Balance	1,810,800,000	1,087,700,000
Total Gains (losses) included in Net Income related to Instruments still held at the  Reporting Date	(725,855,000)	(508,957,000)
Level 3 | Variable annuity
Liabilities:
Beginning Balance	69,728,000	76,119,000
Total Realized and Unrealized Gains Included in Net Income	0	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Net Income	2,675,000	(2,550,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issuances	433,000	0
Settlements	5,863,000	365,000
Transfers in/out of Level 3	0	9,306,000
Other	0	0
Ending Balance	66,973,000	69,728,000
Total Gains (losses) included in Net Income related to Instruments still held at the  Reporting Date	0	0
Level 3 | Other liabilities
Liabilities:
Beginning Balance	1,017,972,000	438,127,000
Total Realized and Unrealized Gains Included in Net Income	502,098,000	108,438,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Net Income	(1,227,953,000)	(617,395,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	0	70,888,000
Sales	0	0
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	0
Other	0	0
Ending Balance	1,743,827,000	1,017,972,000
Total Gains (losses) included in Net Income related to Instruments still held at the  Reporting Date	(725,855,000)	(508,957,000)
Level 3 | Commercial mortgage-backed securities | Available-for-sale securities
Assets:
Beginning Balance	10,029,000	0
Total Realized and Unrealized Gains Included in Net Income	0	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	1,520,000	730,000
Total Realized and Unrealized Losses Included in Net Income	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(797,000)	(91,000)
Purchases	0	9,359,000
Sales	(83,000)	(46,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	21,780,000	95,000
Other	(24,000)	(18,000)
Ending Balance	32,425,000	10,029,000
Total Gains (losses) included in Net Income related to Instruments still held at the  Reporting Date	0	0
Level 3 | Other asset-backed securities | Available-for-sale securities
Assets:
Beginning Balance	421,219,000	421,642,000
Total Realized and Unrealized Gains Included in Net Income	0	904,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	7,559,000	26,034,000
Total Realized and Unrealized Losses Included in Net Income	(38,000)	(71,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(13,213,000)	(8,075,000)
Purchases	0	0
Sales	(1,630,000)	(20,031,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	22,187,000	0
Other	(1,408,000)	816,000
Ending Balance	434,676,000	421,219,000
Total Gains (losses) included in Net Income related to Instruments still held at the  Reporting Date	0	0
Level 3 | Other asset-backed securities | Trading Securities
Assets:
Beginning Balance	65,407,000	26,056,000
Total Realized and Unrealized Gains Included in Net Income	5,979,000	9,295,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Net Income	(8,970,000)	(3,695,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	12,267,000	32,182,000
Sales	(2,646,000)	(24,496,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	(766,000)	26,267,000
Other	129,000	(202,000)
Ending Balance	71,400,000	65,407,000
Total Gains (losses) included in Net Income related to Instruments still held at the  Reporting Date	1,537,000	1,829,000
Level 3 | Corporate securities | Available-for-sale securities
Assets:
Beginning Balance	1,373,714,000	638,276,000
Total Realized and Unrealized Gains Included in Net Income	0	82,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	135,519,000	72,881,000
Total Realized and Unrealized Losses Included in Net Income	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(83,388,000)	(14,827,000)
Purchases	436,539,000	752,929,000
Sales	(562,049,000)	(179,604,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	134,808,000	106,368,000
Other	(3,325,000)	(2,391,000)
Ending Balance	1,431,818,000	1,373,714,000
Total Gains (losses) included in Net Income related to Instruments still held at the  Reporting Date	0	0
Level 3 | Corporate securities | Trading Securities
Assets:
Beginning Balance	11,371,000	6,242,000
Total Realized and Unrealized Gains Included in Net Income	881,000	239,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Net Income	(441,000)	(35,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	8,798,000	1,700,000
Sales	(2,324,000)	(1,035,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	(408,000)	4,363,000
Other	(117,000)	(103,000)
Ending Balance	17,760,000	11,371,000
Total Gains (losses) included in Net Income related to Instruments still held at the  Reporting Date	92,000	35,000
Level 3 | Total fixed maturity securities - available-for-sale
Assets:
Beginning Balance	1,881,740,000	1,092,216,000
Total Realized and Unrealized Gains Included in Net Income	6,860,000	10,520,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	144,598,000	99,645,000
Total Realized and Unrealized Losses Included in Net Income	(9,449,000)	(3,801,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(97,398,000)	(22,993,000)
Purchases	457,604,000	796,170,000
Sales	(568,732,000)	(225,212,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	177,601,000	137,093,000
Other	(4,745,000)	(1,898,000)
Ending Balance	1,988,079,000	1,881,740,000
Total Gains (losses) included in Net Income related to Instruments still held at the  Reporting Date	1,629,000	1,864,000
Level 3 | Total fixed maturity securities - available-for-sale | Available-for-sale securities
Assets:
Beginning Balance	1,804,962,000	1,059,918,000
Total Realized and Unrealized Gains Included in Net Income	0	986,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	144,598,000	99,645,000
Total Realized and Unrealized Losses Included in Net Income	(38,000)	(71,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(97,398,000)	(22,993,000)
Purchases	436,539,000	762,288,000
Sales	(563,762,000)	(199,681,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	178,775,000	106,463,000
Other	(4,757,000)	(1,593,000)
Ending Balance	1,898,919,000	1,804,962,000
Total Gains (losses) included in Net Income related to Instruments still held at the  Reporting Date	0	0
Level 3 | Total fixed maturity securities - available-for-sale | Trading Securities
Assets:
Beginning Balance	76,778,000	32,298,000
Total Realized and Unrealized Gains Included in Net Income	6,860,000	9,534,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Net Income	(9,411,000)	(3,730,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	21,065,000	33,882,000
Sales	(4,970,000)	(25,531,000)
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	(1,174,000)	30,630,000
Other	12,000	(305,000)
Ending Balance	89,160,000	76,778,000
Total Gains (losses) included in Net Income related to Instruments still held at the  Reporting Date	1,629,000	1,864,000
Level 3 | Equity securities
Assets:
Beginning Balance	72,970,000	63,421,000
Total Realized and Unrealized Gains Included in Net Income	1,096,000	(1,829,000)
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	(244,000)
Total Realized and Unrealized Losses Included in Net Income	(3,000)	(18,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	26,944,000	9,567,000
Sales	(5,000,000)	2,073,000
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	5,220,000	0
Other	0	0
Ending Balance	101,227,000	72,970,000
Total Gains (losses) included in Net Income related to Instruments still held at the  Reporting Date	314,000	426,000
Level 3 | Other long-term investments
Assets:
Beginning Balance	209,843,000	151,342,000
Total Realized and Unrealized Gains Included in Net Income	338,541,000	90,078,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Net Income	(228,391,000)	(31,448,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	40,831,000	1,579,000
Sales	(134,757,000)	0
Issuances	0	0
Settlements	(4,072,000)	(1,708,000)
Transfers in/out of Level 3	0	0
Other	0	0
Ending Balance	221,995,000	209,843,000
Total Gains (losses) included in Net Income related to Instruments still held at the  Reporting Date	86,700,000	56,922,000
Level 3 | Short-term investments
Assets:
Beginning Balance	0	0
Total Realized and Unrealized Gains Included in Net Income	0	0
Total Realized and Unrealized Gains Included in Other Comprehensive Income	0	0
Total Realized and Unrealized Losses Included in Net Income	0	0
Total Realized and Unrealized Losses Included in Other Comprehensive Income	0	0
Purchases	0	0
Sales	0	0
Issuances	0	0
Settlements	0	0
Transfers in/out of Level 3	0	0
Other	0	0
Ending Balance	0	0
Total Gains (losses) included in Net Income related to Instruments still held at the  Reporting Date	0	0
Level 3 | Total investments
Assets:
Beginning Balance	2,164,553,000	1,306,979,000
Total Realized and Unrealized Gains Included in Net Income	346,497,000	98,769,000
Total Realized and Unrealized Gains Included in Other Comprehensive Income	144,598,000	99,401,000
Total Realized and Unrealized Losses Included in Net Income	(237,843,000)	(35,267,000)
Total Realized and Unrealized Losses Included in Other Comprehensive Income	(97,398,000)	(22,993,000)
Purchases	525,379,000	807,316,000
Sales	(708,489,000)	(223,139,000)
Transfers in/out of Level 3	182,821,000	137,093,000
Other	(4,745,000)	(1,898,000)
Ending Balance	2,311,301,000	2,164,553,000
Total Gains (losses) included in Net Income related to Instruments still held at the  Reporting Date	$ 88,643,000	$ 59,212,000