INVESTMENT OPERATIONS - Additional Information (Details)	9 Months Ended	12 Months Ended
Sep. 30, 2018 USD ($) position subsidiary	Dec. 31, 2017 USD ($) subsidiary
Investment [Line Items]
Unrealized loss, greater than 12 months	$ 1,376,340,000	$ 601,185,000
Underlying collateral, percentage guaranteed by the Federal Family Education Loan Program		97.00%
Total positions that were in an unrealized loss position \| position	4,367
Fixed maturity securities - available-for-sale	$ 50,019,939,000	$ 39,488,434,000
Available-for-sale, amortized cost	52,187,516,000	39,455,935,000
Fixed maturity securities - trading	2,531,422,000	$ 2,725,694,000
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	1,700,000,000
Available-for-sale, amortized cost	1,700,000,000
Fixed maturity securities - trading	175,600,000
Securities not publicly traded	263,100,000
Residential mortgage-backed securities
Investment [Line Items]
Unrealized loss, greater than 12 months	24,692,000	$ 13,064,000
Fixed maturity securities - available-for-sale	3,406,661,000	2,318,493,000
Available-for-sale, amortized cost	3,506,384,000	2,321,811,000
Fixed maturity securities - trading	254,037,000	259,694,000
Commercial mortgage-backed securities
Investment [Line Items]
Unrealized loss, greater than 12 months	40,932,000	21,031,000
Fixed maturity securities - available-for-sale	2,234,378,000	1,860,488,000
Available-for-sale, amortized cost	2,314,831,000	1,885,109,000
Fixed maturity securities - trading	135,403,000	146,804,000
Other asset-backed securities
Investment [Line Items]
Unrealized loss, greater than 12 months	$ 7,251,000	5,441,000
Underlying collateral, percentage guaranteed by the Federal Family Education Loan Program	97.00%
Fixed maturity securities - available-for-sale	$ 1,454,135,000	1,249,549,000
Available-for-sale, amortized cost	1,456,768,000	1,234,376,000
Fixed maturity securities - trading	111,770,000	138,097,000
U.S. government-related securities
Investment [Line Items]
Unrealized loss, greater than 12 months	58,741,000	31,489,000
Fixed maturity securities - available-for-sale	1,580,979,000	1,223,252,000
Available-for-sale, amortized cost	1,653,626,000	1,255,244,000
Fixed maturity securities - trading	58,081,000	27,234,000
Other government-related securities
Investment [Line Items]
Unrealized loss, greater than 12 months	14,230,000	4,779,000
Fixed maturity securities - available-for-sale	527,646,000	285,233,000
Available-for-sale, amortized cost	546,356,000	280,780,000
Fixed maturity securities - trading	43,030,000	63,925,000
States, municipals, and political subdivisions
Investment [Line Items]
Unrealized loss, greater than 12 months	88,908,000	43,875,000
Fixed maturity securities - available-for-sale	3,558,073,000	1,741,645,000
Available-for-sale, amortized cost	3,691,930,000	1,770,299,000
Fixed maturity securities - trading	296,618,000	326,925,000
Corporate securities
Investment [Line Items]
Unrealized loss, greater than 12 months	1,136,265,000	471,752,000
Fixed maturity securities - available-for-sale	36,860,207,000	29,537,546,000
Available-for-sale, amortized cost	38,612,538,000	29,446,365,000
Fixed maturity securities - trading	1,569,652,000	1,698,183,000
Fixed maturities
Investment [Line Items]
Fixed maturity securities - available-for-sale	49,709,218,000	38,307,297,000
Available-for-sale, amortized cost	51,876,795,000	38,288,346,000
Fixed maturity securities - trading	2,471,756,000	2,664,189,000
Gross unrecognized holding gains	1,728,000	76,586,000
Gross unrecognized holding losses	$ 77,492,000	$ 19,163,000

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