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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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INVESTMENT OPERATIONS - Additional Information (Details)		3 Months Ended
	Jan. 15, 2016 note	Mar. 31, 2019 USD ($) position subsidiary	Dec. 31, 2018 USD ($) subsidiary
Investment [Line Items]
Gross unrealized loss greater than twelve months		$ 1,204,876,000	$ 1,658,939,000
Total positions that were in an unrealized loss position \| position		2,473
Fair Value		$ 52,066,902,000	50,264,864,000
Available-for-sale securities, amortized cost		52,656,572,000	52,826,470,000
Trading securities		$ 2,537,299,000	$ 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 \| note	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		120,700,000
Securities not publicly traded		264,200,000
Residential mortgage-backed securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		27,634,000	$ 30,894,000
Trading securities		213,259,000	241,836,000
Commercial mortgage-backed securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		26,608,000	50,659,000
Trading securities		209,482,000	188,925,000
Other asset-backed securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		$ 3,718,000	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		$ 149,541,000	159,907,000
U.S. government-related securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		29,772,000	41,054,000
Trading securities		59,627,000	59,794,000
Other government-related securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		10,604,000	18,583,000
Trading securities		23,640,000	44,207,000
States, municipals, and political subdivisions
Investment [Line Items]
Gross unrealized loss greater than twelve months		24,887,000	91,722,000
Trading securities		292,796,000	286,413,000
Corporate securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		1,077,532,000	1,414,499,000
Trading securities		1,533,256,000	1,423,833,000
Fixed maturities
Investment [Line Items]
Fair Value		51,283,891,000
Available-for-sale securities, amortized cost		51,873,561,000
Trading securities		2,493,461,000	2,416,192,000
Gross unrecognized holding gains		43,568,000	0
Gross unrecognized holding losses		$ 56,483,000	$ 86,264,000