INVESTMENT OPERATIONS - Additional Information (Details)		3 Months Ended		9 Months Ended		12 Months Ended
	Jan. 15, 2016 note	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 twelve months		$ 624,633,000		$ 624,633,000		$ 1,196,195,000
Total positions that were in an unrealized loss position | position		1,742		1,742
Fair Value		$ 39,356,087,000		$ 39,356,087,000		36,570,665,000
Available-for-sale securities, amortized cost		39,651,286,000		39,651,286,000		$ 38,233,986,000
Other-than-temporary impairments on held-to-maturity securities		$ 0	$ 0	$ 0	$ 0
Red Mountain and Steel City
Investment [Line Items]
Number of wholly owned subsidiaries that were determined to be VIEs | subsidiary		2		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		$ 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		$ 10,000
Payments made				0
Number of notes issued | note	3
Below investment grade
Investment [Line Items]
Trading securities		240,700,000		240,700,000
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		338,400,000		338,400,000
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 Company intended to sell or expected to be required to sell		0	0	0	0
Fair Value		38,096,737,000		38,096,737,000		35,543,477,000
Available-for-sale securities, amortized cost		38,409,559,000		38,409,559,000		37,192,864,000
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]
Trading securities		5,100,000		5,100,000		7,100,000
Other-than-temporary impairments related to fixed maturities or equity securities that the Company intended to sell or expected to be required to sell		0	$ 0	0	$ 0
Gross unrealized loss greater than twelve months		5,046,000		5,046,000		6,412,000
Fair Value		776,427,000		776,427,000		747,406,000
Available-for-sale securities, amortized cost		758,804,000		758,804,000		761,340,000
Short-term investments
Investment [Line Items]
Trading securities		44,300,000		44,300,000		52,600,000
Fair Value		482,923,000		482,923,000		279,782,000
Available-for-sale securities, amortized cost		482,923,000		482,923,000		279,782,000
Residential mortgage-backed securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		2,137,000		2,137,000		4,117,000
Fair Value		2,192,254,000		2,192,254,000		1,898,483,000
Available-for-sale securities, amortized cost		2,187,460,000		2,187,460,000		1,913,413,000
Commercial mortgage-backed securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		4,634,000		4,634,000		4,013,000
Fair Value		1,921,429,000		1,921,429,000		1,811,470,000
Available-for-sale securities, amortized cost		1,939,832,000		1,939,832,000		1,850,620,000
Other asset-backed securities
Investment [Line Items]
Gross unrealized loss greater than twelve months		8,424,000		$ 8,424,000		$ 11,407,000
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%
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 twelve months		6,548,000		6,548,000		11,890,000
Fair Value		254,336,000		254,336,000		239,921,000
Available-for-sale securities, amortized cost		253,026,000		253,026,000		253,182,000
States, municipals, and political subdivisions
Investment [Line Items]
Gross unrealized loss greater than twelve months		36,880,000		36,880,000		41,749,000
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 twelve months		553,638,000		553,638,000		1,114,729,000
Fair Value		29,449,369,000		29,449,369,000		27,371,565,000
Available-for-sale securities, amortized cost		$ 29,673,641,000		$ 29,673,641,000		$ 28,801,768,000