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INVESTMENT OPERATIONS - Additional Information (Details)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($) position subsidiary	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) position subsidiary	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) subsidiary
Investment [Line Items]
Gross unrealized loss, greater than 12 months	$ 624,549,000		$ 624,549,000		$ 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	1,742		1,742
Available-for-sale securities, fair value	$ 39,007,653,000		$ 39,007,653,000		$ 36,328,836,000
Available-for-sale securities, amortized cost	39,306,705,000		39,306,705,000		$ 37,991,304,000
Other-than-temporary impairments on held-to-maturity securities	$ 0	$ 0	0	$ 0
Payments made			$ 0
Red Mountain LLC
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
Below investment grade
Investment [Line Items]
Trading securities	240,700,000		240,700,000
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	337,300,000		337,300,000
Fixed maturities
Investment [Line Items]
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
Trading securities	2,700,000,000		2,700,000,000		$ 2,600,000,000
Available-for-sale securities, fair value	37,890,158,000		37,890,158,000		35,356,717,000
Available-for-sale securities, amortized cost	38,206,780,000		38,206,780,000		37,005,251,000
Gross unrecognized holding gains	82,913,000		82,913,000		30,385,000
Gross unrecognized holding losses	17,305,000		17,305,000		67,222,000
Equity securities
Investment [Line Items]
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
Trading securities	5,100,000		5,100,000		7,100,000
Gross unrealized loss, greater than 12 months	5,046,000		5,046,000		6,412,000
Available-for-sale securities, fair value	739,066,000		739,066,000		708,934,000
Available-for-sale securities, amortized cost	721,496,000		721,496,000		722,868,000
Short-term investments
Investment [Line Items]
Trading securities	44,300,000		44,300,000		52,600,000
Available-for-sale securities, fair value	378,429,000		378,429,000		263,185,000
Available-for-sale securities, amortized cost	378,429,000		378,429,000		263,185,000
Residential mortgage-backed securities
Investment [Line Items]
Gross unrealized loss, greater than 12 months	2,137,000		2,137,000		4,117,000
Available-for-sale securities, fair value	2,183,369,000		2,183,369,000		1,889,607,000
Available-for-sale securities, amortized cost	2,178,344,000		2,178,344,000		1,904,165,000
Commercial mortgage-backed securities
Investment [Line Items]
Gross unrealized loss, greater than 12 months	4,587,000		4,587,000		4,013,000
Available-for-sale securities, fair value	1,892,053,000		1,892,053,000		1,782,497,000
Available-for-sale securities, amortized cost	1,909,921,000		1,909,921,000		1,820,644,000
Other asset-backed securities
Investment [Line Items]
Gross unrealized loss, greater than 12 months	8,424,000		8,424,000		11,407,000
Available-for-sale securities, fair value	1,190,667,000		1,190,667,000		1,211,533,000
Available-for-sale securities, amortized cost	1,186,463,000		1,186,463,000		1,210,490,000
Other government-related securities
Investment [Line Items]
Gross unrealized loss, greater than 12 months	6,548,000		6,548,000		11,890,000
Available-for-sale securities, fair value	252,280,000		252,280,000		237,926,000
Available-for-sale securities, amortized cost	251,040,000		251,040,000		251,197,000
States, municipals, and political subdivisions
Investment [Line Items]
Gross unrealized loss, greater than 12 months	36,880,000		36,880,000		41,749,000
Available-for-sale securities, fair value	1,704,009,000		1,704,009,000		1,656,503,000
Available-for-sale securities, amortized cost	1,756,865,000		1,756,865,000		1,760,837,000
Corporate securities
Investment [Line Items]
Gross unrealized loss, greater than 12 months	553,601,000		553,601,000		1,114,635,000
Available-for-sale securities, fair value	29,283,107,000		29,283,107,000		27,224,649,000
Available-for-sale securities, amortized cost	$ 29,511,875,000		$ 29,511,875,000		$ 28,655,364,000