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INVESTMENT OPERATIONS - Additional Information (Details)		9 Months Ended
	Jan. 15, 2016	Sep. 30, 2019 USD ($) subsidiary position	Dec. 31, 2018 USD ($) subsidiary
Investment [Line Items]
Gross unrealized loss greater than twelve months		$ 296,566,000	$ 1,658,939,000
Total positions that were in an unrealized loss position \| position		809
Fair Value		$ 65,643,018,000	50,264,864,000
Available-for-sale securities, amortized cost		62,622,141,000	52,826,470,000
Trading securities		$ 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		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	3
Below investment grade
Investment [Line Items]
Fair Value		1,600,000,000
Available-for-sale securities, amortized cost		1,700,000,000
Trading securities		113,500,000
Securities not publicly traded		235,200,000
Residential mortgage-backed securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		2,955,000	$ 30,894,000
Trading securities		202,963,000	241,836,000
Commercial mortgage-backed securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		1,041,000	50,659,000
Trading securities		208,935,000	188,925,000
Other asset-backed securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		$ 5,507,000	4,435,000
Percentage of underlying collateral of student-loan backed auction rate securities guaranteed by the Federal Family Education Loan Program (FFELP), minimum		97.00%
Trading securities		$ 137,732,000	159,907,000
U.S. government-related securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		1,725,000	41,054,000
Trading securities		48,112,000	59,794,000
Other government-related securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		1,142,000	18,583,000
Trading securities		26,792,000	44,207,000
States, municipals, and political subdivisions
Investment [Line Items]
Gross unrealized loss greater than twelve months		300,000	91,722,000
Trading securities		300,332,000	286,413,000
Corporate securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		279,893,000	1,414,499,000
Trading securities		1,599,453,000	1,423,833,000
Fixed maturities
Investment [Line Items]
Fair Value		64,144,082,000
Available-for-sale securities, amortized cost		61,123,205,000
Trading securities		2,536,514,000	2,416,192,000
Gross unrecognized holding gains		141,022,000	0
Gross unrecognized holding losses		$ 0	$ 86,264,000