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INVESTMENT OPERATIONS - Additional Information (Details)		6 Months Ended
	Jan. 15, 2016	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($) subsidiary
Investment [Line Items]
Gross unrealized loss greater than twelve months		$ 556,747,000	$ 1,658,939,000
Total positions that were in an unrealized loss position		1,322
Fair Value		$ 63,347,339,000	50,264,864,000
Available-for-sale securities, amortized cost		61,985,929,000	52,826,470,000
Trading securities		$ 2,592,645,000	$ 2,457,010,000
Red Mountain and Steel City
Investment [Line Items]
Number of wholly owned subsidiaries that were determined to be VIEs		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		119,900,000
Securities not publicly traded		275,600,000
Residential mortgage-backed securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		8,474,000	$ 30,894,000
Trading securities		203,455,000	241,836,000
Commercial mortgage-backed securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		3,888,000	50,659,000
Trading securities		204,445,000	188,925,000
Other asset-backed securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		$ 2,595,000	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		$ 147,997,000	159,907,000
U.S. government-related securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		8,690,000	41,054,000
Trading securities		51,080,000	59,794,000
Other government-related securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		3,970,000	18,583,000
Trading securities		24,342,000	44,207,000
States, municipals, and political subdivisions
Investment [Line Items]
Gross unrealized loss greater than twelve months		5,838,000	91,722,000
Trading securities		293,560,000	286,413,000
Corporate securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		519,534,000	1,414,499,000
Trading securities		1,565,561,000	1,423,833,000
Fixed maturities
Investment [Line Items]
Fair Value		62,208,408,000
Available-for-sale securities, amortized cost		60,846,998,000
Trading securities		2,502,248,000	2,416,192,000
Gross unrecognized holding gains		78,860,000	0
Gross unrecognized holding losses		$ 14,249,000	$ 86,264,000