INVESTMENT OPERATIONS - Additional Information (Details)	3 Months Ended	12 Months Ended
Mar. 31, 2017 USD ($) position subsidiary	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($) subsidiary
Investment [Line Items]
Gross unrealized loss greater than twelve months	$ 1,004,007,000		$ 1,196,195,000
Total positions that were in an unrealized loss position \| position	2,171
Fair Value	$ 36,966,122,000		36,570,665,000
Available-for-sale securities, amortized cost	38,262,236,000		$ 38,233,986,000
Other-than-temporary impairments on held-to-maturity securities	$ 0	$ 0
Red Mountain and Steel City
Investment [Line Items]
Number of wholly owned subsidiaries that were determined to be VIEs \| subsidiary	2		2
Red Mountain
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
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
Below investment grade
Investment [Line Items]
Trading securities	259,800,000
Fair Value	2,000,000,000
Available-for-sale securities, amortized cost	2,000,000,000
Securities not publicly traded	361,000,000
Fixed maturities
Investment [Line Items]
Trading securities	2,600,000,000		$ 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
Fair Value	35,931,511,000		35,543,477,000
Available-for-sale securities, amortized cost	37,236,105,000		37,192,864,000
Gross unrealized gains	43,243,000		30,385,000
Gross unrealized losses	55,005,000		67,222,000
Equity securities
Investment [Line Items]
Trading securities	5,500,000		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
Gross unrealized loss greater than twelve months	5,524,000		6,412,000
Fair Value	786,693,000		747,406,000
Available-for-sale securities, amortized cost	778,213,000		761,340,000
Short-term investments
Investment [Line Items]
Trading securities	51,200,000		52,600,000
Fair Value	247,918,000		279,782,000
Available-for-sale securities, amortized cost	247,918,000		279,782,000
Residential mortgage-backed securities
Investment [Line Items]
Gross unrealized loss greater than twelve months	4,034,000		4,117,000
Fair Value	1,970,192,000		1,898,483,000
Available-for-sale securities, amortized cost	1,988,289,000		1,913,413,000
Commercial mortgage-backed securities
Investment [Line Items]
Gross unrealized loss greater than twelve months	4,324,000		4,013,000
Fair Value	1,814,663,000		1,811,470,000
Available-for-sale securities, amortized cost	1,850,816,000		1,850,620,000
Other asset-backed securities
Investment [Line Items]
Gross unrealized loss greater than twelve months	$ 11,312,000		$ 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	$ 1,182,450,000		$ 1,211,533,000
Available-for-sale securities, amortized cost	1,177,426,000		1,210,490,000
Other government-related securities
Investment [Line Items]
Gross unrealized loss greater than twelve months	10,791,000		11,890,000
Fair Value	245,201,000		239,921,000
Available-for-sale securities, amortized cost	253,129,000		253,182,000
States, municipals, and political subdivisions
Investment [Line Items]
Gross unrealized loss greater than twelve months	42,740,000		41,749,000
Fair Value	1,672,724,000		1,656,503,000
Available-for-sale securities, amortized cost	1,776,716,000		1,760,837,000
Corporate securities
Investment [Line Items]
Gross unrealized loss greater than twelve months	923,324,000		1,114,729,000
Fair Value	27,682,212,000		27,371,565,000
Available-for-sale securities, amortized cost	$ 28,785,229,000		$ 28,801,768,000

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