INVESTMENT OPERATIONS - Additional Information (Details)	3 Months Ended
Mar. 31, 2019 USD ($) position subsidiary	Dec. 31, 2018 USD ($) subsidiary
Investment [Line Items]
Unrealized loss, greater than 12 months	$ 1,202,457,000	$ 1,654,537,000
Total positions that were in an unrealized loss position \| position	2,473
Fixed maturity securities - available-for-sale	$ 51,688,233,000	49,898,408,000
Available-for-sale, amortized cost	52,278,016,000	52,452,334,000
Fixed maturity securities - trading	2,537,299,000	$ 2,457,010,000
Payments made	$ 0
Red Mountain, LLC
Investment [Line Items]
Number of wholly owned subsidiaries that were determined to be VIEs \| subsidiary	1	1
Ownership interest through an affiliate (as a percent)	100.00%
Investment to which risk of loss related to the VIE is limited	$ 10,000
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	120,700,000
Securities not publicly traded	264,200,000
Residential mortgage-backed securities
Investment [Line Items]
Unrealized loss, greater than 12 months	27,543,000	$ 30,633,000
Fixed maturity securities - available-for-sale	3,899,531,000	3,602,991,000
Available-for-sale, amortized cost	3,871,471,000	3,641,678,000
Fixed maturity securities - trading	213,259,000	241,836,000
Commercial mortgage-backed securities
Investment [Line Items]
Unrealized loss, greater than 12 months	26,026,000	49,602,000
Fixed maturity securities - available-for-sale	2,324,098,000	2,266,387,000
Available-for-sale, amortized cost	2,334,211,000	2,319,476,000
Fixed maturity securities - trading	209,482,000	188,925,000
Other asset-backed securities
Investment [Line Items]
Unrealized loss, greater than 12 months	$ 3,718,000	4,435,000
Underlying collateral, percentage guaranteed by the Federal Family Education Loan Program	97.00%
Fixed maturity securities - available-for-sale	$ 1,362,611,000	1,391,893,000
Available-for-sale, amortized cost	1,359,813,000	1,410,059,000
Fixed maturity securities - trading	149,541,000	159,907,000
U.S. government-related securities
Investment [Line Items]
Unrealized loss, greater than 12 months	29,772,000	41,054,000
Fixed maturity securities - available-for-sale	1,411,106,000	1,614,505,000
Available-for-sale, amortized cost	1,439,136,000	1,658,433,000
Fixed maturity securities - trading	59,627,000	59,794,000
Other government-related securities
Investment [Line Items]
Unrealized loss, greater than 12 months	10,604,000	18,583,000
Fixed maturity securities - available-for-sale	522,125,000	514,036,000
Available-for-sale, amortized cost	521,713,000	543,534,000
Fixed maturity securities - trading	23,640,000	44,207,000
States, municipals, and political subdivisions
Investment [Line Items]
Unrealized loss, greater than 12 months	24,887,000	91,722,000
Fixed maturity securities - available-for-sale	3,691,664,000	3,588,841,000
Available-for-sale, amortized cost	3,632,880,000	3,682,037,000
Fixed maturity securities - trading	292,796,000	286,413,000
Corporate securities
Investment [Line Items]
Unrealized loss, greater than 12 months	1,075,786,000	1,411,415,000
Fixed maturity securities - available-for-sale	37,786,258,000	36,201,578,000
Available-for-sale, amortized cost	38,424,196,000	38,467,380,000
Fixed maturity securities - trading	1,533,256,000	1,423,833,000
Fixed maturities
Investment [Line Items]
Fixed maturity securities - available-for-sale	51,081,216,000	49,263,033,000
Available-for-sale, amortized cost	51,670,999,000	51,816,959,000
Fixed maturity securities - trading	2,493,461,000	2,416,192,000
Gross unrecognized holding gains	43,568,000	0
Gross unrecognized holding losses	$ 56,483,000	$ 86,264,000

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