INVESTMENT OPERATIONS - Additional Information (Details)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017 USD ($) subsidiary position	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) subsidiary position	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) subsidiary
Investment [Line Items]
Gross unrealized loss, greater than 12 months	$ 704,685,000		$ 704,685,000		$ 1,196,101,000
Underlying collateral, Percentage guaranteed by the Federal Family Education Loan Program			97.00%		97.00%
Total positions that were in an unrealized loss position | position	1,846		1,846
Available-for-sale securities, fair value	$ 37,543,164,000		$ 37,543,164,000		$ 36,328,836,000
Available-for-sale securities, amortized cost	38,098,486,000		38,098,486,000		$ 37,991,304,000
Other-than-temporary impairments on held-to-maturity securities	$ 0	$ 0	0	$ 0
Payments made			$ 0
Red Mountain
Investment [Line Items]
Number of wholly owned subsidiaries that were determined to be VIEs | subsidiary	1		1		1
Ownership interest through an affiliate (as a percent)			100.00%
Investment to which risk of loss related to the VIE is limited	$ 10,000		$ 10,000
External Credit Rating, Non Investment Grade
Investment [Line Items]
Trading securities	240,200,000		240,200,000
Available-for-sale securities, fair value	1,900,000,000		1,900,000,000
Available-for-sale securities, amortized cost	1,900,000,000		1,900,000,000
Securities not publicly traded	341,500,000		341,500,000
Fixed maturities
Investment [Line Items]
Trading securities	2,700,000,000		2,700,000,000		$ 2,600,000,000
Other-than-temporary impairments related to fixed maturities or equity securities that the entity intended to sell or expected to be required to sell	0	0	0	0
Available-for-sale securities, fair value	36,481,926,000		36,481,926,000		35,356,717,000
Available-for-sale securities, amortized cost	37,056,222,000		37,056,222,000		37,005,251,000
Gross Unrealized Gains	80,605,000		80,605,000		30,385,000
Gross Unrealized Losses	22,575,000		22,575,000		67,222,000
Equity securities
Investment [Line Items]
Trading securities	5,200,000		5,200,000		7,100,000
Other-than-temporary impairments related to fixed maturities or equity securities that the entity intended to sell or expected to be required to sell	0	$ 0	0	$ 0
Gross unrealized loss, greater than 12 months	4,670,000		4,670,000		6,412,000
Available-for-sale securities, fair value	744,954,000		744,954,000		708,934,000
Available-for-sale securities, amortized cost	725,980,000		725,980,000		722,868,000
Short-term investments
Investment [Line Items]
Trading securities	45,800,000		45,800,000		52,600,000
Available-for-sale securities, fair value	316,284,000		316,284,000		263,185,000
Available-for-sale securities, amortized cost	316,284,000		316,284,000		263,185,000
Residential mortgage-backed securities
Investment [Line Items]
Gross unrealized loss, greater than 12 months	2,178,000		2,178,000		4,117,000
Available-for-sale securities, fair value	1,992,315,000		1,992,315,000		1,889,607,000
Available-for-sale securities, amortized cost	1,997,251,000		1,997,251,000		1,904,165,000
Commercial mortgage-backed securities
Investment [Line Items]
Gross unrealized loss, greater than 12 months	3,658,000		3,658,000		4,013,000
Available-for-sale securities, fair value	1,802,778,000		1,802,778,000		1,782,497,000
Available-for-sale securities, amortized cost	1,822,565,000		1,822,565,000		1,820,644,000
Other asset-backed securities
Investment [Line Items]
Gross unrealized loss, greater than 12 months	11,263,000		11,263,000		11,407,000
Available-for-sale securities, fair value	1,182,003,000		1,182,003,000		1,211,533,000
Available-for-sale securities, amortized cost	1,179,284,000		1,179,284,000		1,210,490,000
Other government-related securities
Investment [Line Items]
Gross unrealized loss, greater than 12 months	7,694,000		7,694,000		11,890,000
Available-for-sale securities, fair value	249,458,000		249,458,000		237,926,000
Available-for-sale securities, amortized cost	251,130,000		251,130,000		251,197,000
States, municipals, and political subdivisions
Investment [Line Items]
Gross unrealized loss, greater than 12 months	36,331,000		36,331,000		41,749,000
Available-for-sale securities, fair value	1,689,321,000		1,689,321,000		1,656,503,000
Available-for-sale securities, amortized cost	1,755,780,000		1,755,780,000		1,760,837,000
Corporate securities
Investment [Line Items]
Gross unrealized loss, greater than 12 months	636,887,000		636,887,000		1,114,635,000
Available-for-sale securities, fair value	28,176,875,000		28,176,875,000		27,224,649,000
Available-for-sale securities, amortized cost	$ 28,634,563,000		$ 28,634,563,000		$ 28,655,364,000