INVESTMENT OPERATIONS - Additional Information (Details)		6 Months Ended	12 Months Ended
	Jan. 15, 2016 note	Jun. 30, 2018 USD ($) position subsidiary	Dec. 31, 2017 USD ($) subsidiary
Investment [Line Items]
Gross unrealized loss greater than twelve months		$ 1,298,764,000	$ 602,685,000
Total positions that were in an unrealized loss position | position		3,951
Available-for-sale securities		$ 49,915,910,000	39,819,928,000
Available-for-sale securities, amortized cost		51,798,849,000	39,783,880,000
Trading securities		$ 2,550,330,000	$ 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]
Available-for-sale securities		1,900,000,000
Available-for-sale securities, amortized cost		2,000,000,000
Trading securities		207,200,000
Securities not publicly traded		346,900,000
Residential mortgage-backed securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		19,832,000	$ 13,362,000
Available-for-sale securities		3,243,980,000	2,327,212,000
Available-for-sale securities, amortized cost		3,297,665,000	2,330,832,000
Trading securities		259,755,000	259,694,000
Commercial mortgage-backed securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		39,381,000	21,594,000
Available-for-sale securities		2,227,402,000	1,889,822,000
Available-for-sale securities, amortized cost		2,298,945,000	1,914,998,000
Trading securities		141,469,000	146,804,000
Other asset-backed securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		$ 7,560,000	$ 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%
Available-for-sale securities		$ 1,423,561,000	$ 1,249,549,000
Available-for-sale securities, amortized cost		1,408,987,000	1,234,376,000
Trading securities		114,507,000	138,097,000
U.S. government-related securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		50,863,000	31,489,000
Available-for-sale securities		1,492,036,000	1,223,252,000
Available-for-sale securities, amortized cost		1,549,033,000	1,255,244,000
Trading securities		36,575,000	27,234,000
Other government-related securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		11,492,000	4,778,000
Available-for-sale securities		401,730,000	287,282,000
Available-for-sale securities, amortized cost		417,232,000	282,767,000
Trading securities		40,274,000	63,925,000
States, municipals, and political subdivisions
Investment [Line Items]
Gross unrealized loss greater than twelve months		68,578,000	43,874,000
Available-for-sale securities		3,633,803,000	1,741,645,000
Available-for-sale securities, amortized cost		3,699,964,000	1,770,299,000
Trading securities		314,722,000	326,925,000
Corporate securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		1,098,311,000	472,394,000
Available-for-sale securities		36,743,236,000	29,702,010,000
Available-for-sale securities, amortized cost		38,372,075,000	29,606,484,000
Trading securities		1,585,290,000	1,698,183,000
Fixed maturities
Investment [Line Items]
Available-for-sale securities		49,255,324,000	38,511,863,000
Available-for-sale securities, amortized cost		51,138,263,000	38,489,362,000
Trading securities		2,495,881,000	2,664,189,000
Gross unrecognized holding losses		94,788,000	19,163,000
Gross unrecognized holding gains		$ 0	$ 76,586,000