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INVESTMENT OPERATIONS - Additional Information (Details)		3 Months Ended		12 Months Ended
	Jan. 15, 2016 note	Mar. 31, 2018 USD ($) position subsidiary	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) subsidiary
Investment [Line Items]
Gross unrealized loss greater than twelve months		$ 1,031,096,000		$ 602,685,000
Total positions that were in an unrealized loss position \| position		2,428
Fair Value		$ 37,807,698,000		39,819,928,000
Available-for-sale securities, amortized cost		38,949,464,000		39,783,880,000
Trading securities		2,662,999,000		$ 2,725,694,000
Other-than-temporary impairments on held-to-maturity securities		$ 0	$ 0
Red Mountain and Steel City
Investment [Line Items]
Number of wholly owned subsidiaries that were determined to be VIEs \| subsidiary		2		2
Red Mountain LLC
Investment [Line Items]
Ownership through an affiliate (as a percent)		100.00%
Risk of loss related to the VIE limited to the entity's investment		$ 10,000
Payments made		$ 0
Steel City LLC
Investment [Line Items]
Ownership through an affiliate (as a percent)		100.00%
Risk of loss related to the VIE limited to the entity's investment		$ 10,000
Payments made		0
Number of notes issued \| note	3
Below investment grade
Investment [Line Items]
Fair Value		1,800,000,000
Available-for-sale securities, amortized cost		1,900,000,000
Trading securities		217,200,000
Securities not publicly traded		307,500,000
Fixed maturities
Investment [Line Items]
Other-than-temporary impairments related to fixed maturities or equity securities that the Company intended to sell or expected to be required to sell		0	0
Fair Value		37,436,408,000		$ 38,511,863,000
Available-for-sale securities, amortized cost		38,578,174,000		38,489,362,000
Trading securities		2,587,142,000		2,664,189,000
Gross unrecognized holding gains		30,310,000		76,586,000
Gross unrecognized holding losses		56,007,000		19,163,000
Residential mortgage-backed securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		18,165,000		13,362,000
Fair Value		2,394,370,000		2,327,212,000
Available-for-sale securities, amortized cost		2,430,278,000		2,330,832,000
Trading securities		265,547,000		259,694,000
Commercial mortgage-backed securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		34,463,000		21,594,000
Fair Value		1,841,506,000		1,889,822,000
Available-for-sale securities, amortized cost		1,899,618,000		1,914,998,000
Trading securities		146,633,000		146,804,000
Other asset-backed securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		$ 7,869,000		$ 5,441,000
Percentage of underlying collateral of student-loan backed auction rate securities guaranteed by the Federal Family Education Loan Program (FFELP), minimum		97.00%		97.00%
Fair Value		$ 1,244,652,000		$ 1,249,549,000
Available-for-sale securities, amortized cost		1,238,929,000		1,234,376,000
Trading securities		129,714,000		138,097,000
U.S. government-related securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		46,893,000		31,489,000
Fair Value		1,327,800,000		1,223,252,000
Available-for-sale securities, amortized cost		1,377,633,000		1,255,244,000
Trading securities		37,575,000		27,234,000
Other government-related securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		7,405,000		4,778,000
Fair Value		277,459,000		287,282,000
Available-for-sale securities, amortized cost		282,664,000		282,767,000
Trading securities		40,368,000		63,925,000
States, municipals, and political subdivisions
Investment [Line Items]
Gross unrealized loss greater than twelve months		62,719,000		43,874,000
Fair Value		1,700,279,000		1,741,645,000
Available-for-sale securities, amortized cost		1,767,604,000		1,770,299,000
Trading securities		318,142,000		326,925,000
Corporate securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		850,861,000		472,394,000
Fair Value		28,559,773,000		29,702,010,000
Available-for-sale securities, amortized cost		29,487,086,000		29,606,484,000
Trading securities		1,645,899,000		1,698,183,000
Equity securities
Investment [Line Items]
Other-than-temporary impairments related to fixed maturities or equity securities that the Company intended to sell or expected to be required to sell		0	$ 0
Gross unrealized loss greater than twelve months				7,370,000
Fair Value				749,116,000
Available-for-sale securities, amortized cost				735,569,000
Trading securities		$ 5,366,000		$ 5,244,000