INVESTMENT OPERATIONS - Additional Information (Details)	6 Months Ended
Jun. 30, 2019 USD ($) position	Dec. 31, 2018 USD ($) subsidiary
Investment [Line Items]
Unrealized loss, greater than 12 months	$ 555,975,000	$ 1,654,537,000
Total positions that were in an unrealized loss position \| position	1,322
Fixed maturity securities - available-for-sale	$ 62,973,464,000	49,898,408,000
Available-for-sale, amortized cost	61,618,378,000	52,452,334,000
Fixed maturity securities - trading	$ 2,592,645,000	$ 2,457,010,000
Red Mountain and Steel City
Investment [Line Items]
Number of wholly owned subsidiaries that were determined to be VIEs \| subsidiary		1
Red Mountain, LLC
Investment [Line Items]
Ownership interest through an affiliate (as a percent)	100.00%
Investment to which risk of loss related to the VIE is limited	$ 10,000
Payments made	0
Below investment grade
Investment [Line Items]
Fixed maturity securities - available-for-sale	1,600,000,000
Available-for-sale, amortized cost	1,700,000,000
Fixed maturity securities - trading	119,900,000
Securities not publicly traded	275,600,000
Residential mortgage-backed securities
Investment [Line Items]
Unrealized loss, greater than 12 months	8,469,000	$ 30,633,000
Fixed maturity securities - available-for-sale	4,597,444,000	3,602,991,000
Available-for-sale, amortized cost	4,473,723,000	3,641,678,000
Fixed maturity securities - trading	203,455,000	241,836,000
Commercial mortgage-backed securities
Investment [Line Items]
Unrealized loss, greater than 12 months	3,811,000	49,602,000
Fixed maturity securities - available-for-sale	2,665,854,000	2,266,387,000
Available-for-sale, amortized cost	2,618,367,000	2,319,476,000
Fixed maturity securities - trading	204,445,000	188,925,000
Other asset-backed securities
Investment [Line Items]
Unrealized loss, greater than 12 months	$ 2,595,000	4,435,000
Underlying collateral, percentage guaranteed by the Federal Family Education Loan Program	97.00%
Fixed maturity securities - available-for-sale	$ 1,861,598,000	1,391,893,000
Available-for-sale, amortized cost	1,848,061,000	1,410,059,000
Fixed maturity securities - trading	147,997,000	159,907,000
U.S. government-related securities
Investment [Line Items]
Unrealized loss, greater than 12 months	8,690,000	41,054,000
Fixed maturity securities - available-for-sale	1,300,557,000	1,614,505,000
Available-for-sale, amortized cost	1,304,382,000	1,658,433,000
Fixed maturity securities - trading	51,080,000	59,794,000
Other government-related securities
Investment [Line Items]
Unrealized loss, greater than 12 months	3,970,000	18,583,000
Fixed maturity securities - available-for-sale	581,036,000	514,036,000
Available-for-sale, amortized cost	553,779,000	543,534,000
Fixed maturity securities - trading	24,342,000	44,207,000
States, municipals, and political subdivisions
Investment [Line Items]
Unrealized loss, greater than 12 months	5,838,000	91,722,000
Fixed maturity securities - available-for-sale	4,830,966,000	3,588,841,000
Available-for-sale, amortized cost	4,652,897,000	3,682,037,000
Fixed maturity securities - trading	293,560,000	286,413,000
Corporate securities
Investment [Line Items]
Unrealized loss, greater than 12 months	518,844,000	1,411,415,000
Fixed maturity securities - available-for-sale	46,075,152,000	36,201,578,000
Available-for-sale, amortized cost	45,103,765,000	38,467,380,000
Fixed maturity securities - trading	1,565,561,000	1,423,833,000
Fixed maturities
Investment [Line Items]
Fixed maturity securities - available-for-sale	61,997,519,000	49,263,033,000
Available-for-sale, amortized cost	60,642,433,000	51,816,959,000
Fixed maturity securities - trading	2,502,248,000	2,416,192,000
Gross unrecognized holding gains	78,860,000	0
Gross unrecognized holding losses	$ 14,249,000	$ 86,264,000

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