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INVESTMENT OPERATIONS - Additional Information (Details)		9 Months Ended	12 Months Ended
	Jan. 15, 2016 note	Sep. 30, 2018 USD ($) position subsidiary	Dec. 31, 2017 USD ($) subsidiary
Investment [Line Items]
Gross unrealized loss greater than twelve months		$ 1,379,164,000	$ 602,685,000
Total positions that were in an unrealized loss position \| position		4,367
Fair Value		$ 50,385,896,000	39,819,928,000
Available-for-sale securities, amortized cost		52,559,863,000	39,783,880,000
Trading securities		$ 2,531,422,000	$ 2,725,694,000
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		1,700,000,000
Available-for-sale securities, amortized cost		1,700,000,000
Trading securities		175,600,000
Securities not publicly traded		263,100,000
Residential mortgage-backed securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		25,077,000	$ 13,362,000
Trading securities		254,037,000	259,694,000
Commercial mortgage-backed securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		41,983,000	21,594,000
Trading securities		135,403,000	146,804,000
Other asset-backed securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		$ 7,251,000	$ 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%
Trading securities		$ 111,770,000	$ 138,097,000
U.S. government-related securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		58,741,000	31,489,000
Trading securities		58,081,000	27,234,000
Other government-related securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		14,229,000	4,778,000
Trading securities		43,030,000	63,925,000
States, municipals, and political subdivisions
Investment [Line Items]
Gross unrealized loss greater than twelve months		88,908,000	43,874,000
Trading securities		296,618,000	326,925,000
Corporate securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		1,137,654,000	472,394,000
Trading securities		1,569,652,000	1,698,183,000
Fixed maturities
Investment [Line Items]
Fair Value		49,909,708,000
Available-for-sale securities, amortized cost		52,083,675,000
Trading securities		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