INVESTMENT OPERATIONS - Additional Information (Details)	9 Months Ended
Sep. 30, 2019 USD ($) subsidiary position	Dec. 31, 2018 USD ($) subsidiary
Investment [Line Items]
Unrealized loss, greater than 12 months	$ 296,147,000	$ 1,654,537,000
Total positions that were in an unrealized loss position \| position	809
Fixed maturity securities - available-for-sale	$ 64,872,331,000	49,898,408,000
Available-for-sale, amortized cost	61,861,481,000	52,452,334,000
Fixed maturity securities - trading	$ 2,642,144,000	$ 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	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	113,500,000
Securities not publicly traded	235,200,000
Residential mortgage-backed securities
Investment [Line Items]
Unrealized loss, greater than 12 months	2,951,000	$ 30,633,000
Fixed maturity securities - available-for-sale	5,340,663,000	3,602,991,000
Available-for-sale, amortized cost	5,151,583,000	3,641,678,000
Fixed maturity securities - trading	202,963,000	241,836,000
Commercial mortgage-backed securities
Investment [Line Items]
Unrealized loss, greater than 12 months	1,041,000	49,602,000
Fixed maturity securities - available-for-sale	2,714,466,000	2,266,387,000
Available-for-sale, amortized cost	2,627,310,000	2,319,476,000
Fixed maturity securities - trading	208,935,000	188,925,000
Other asset-backed securities
Investment [Line Items]
Unrealized loss, greater than 12 months	$ 5,507,000	4,435,000
Underlying collateral, percentage guaranteed by the Federal Family Education Loan Program	100.00%
Fixed maturity securities - available-for-sale	$ 1,898,476,000	1,391,893,000
Available-for-sale, amortized cost	1,878,663,000	1,410,059,000
Fixed maturity securities - trading	137,732,000	159,907,000
U.S. government-related securities
Investment [Line Items]
Unrealized loss, greater than 12 months	1,725,000	41,054,000
Fixed maturity securities - available-for-sale	1,162,207,000	1,614,505,000
Available-for-sale, amortized cost	1,154,940,000	1,658,433,000
Fixed maturity securities - trading	48,112,000	59,794,000
Other government-related securities
Investment [Line Items]
Unrealized loss, greater than 12 months	1,142,000	18,583,000
Fixed maturity securities - available-for-sale	603,236,000	514,036,000
Available-for-sale, amortized cost	552,445,000	543,534,000
Fixed maturity securities - trading	26,792,000	44,207,000
States, municipals, and political subdivisions
Investment [Line Items]
Unrealized loss, greater than 12 months	300,000	91,722,000
Fixed maturity securities - available-for-sale	4,834,657,000	3,588,841,000
Available-for-sale, amortized cost	4,503,629,000	3,682,037,000
Fixed maturity securities - trading	300,332,000	286,413,000
Corporate securities
Investment [Line Items]
Unrealized loss, greater than 12 months	279,478,000	1,411,415,000
Fixed maturity securities - available-for-sale	47,285,605,000	36,201,578,000
Available-for-sale, amortized cost	44,960,104,000	38,467,380,000
Fixed maturity securities - trading	1,599,453,000	1,423,833,000
Fixed maturities
Investment [Line Items]
Fixed maturity securities - available-for-sale	63,926,872,000	49,263,033,000
Available-for-sale, amortized cost	60,916,022,000	51,816,959,000
Fixed maturity securities - trading	2,536,514,000	2,416,192,000
Gross unrecognized holding gains	141,022,000	0
Gross unrecognized holding losses	$ 0	$ 86,264,000

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