INVESTMENT OPERATIONS (Details 2) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Amortized cost and fair value of the Company's investments classified as available-for-sale
Amortized Cost	$ 24,624,279,000	$ 24,104,126,000
Gross Unrealized Gains	3,054,798,000	3,251,213,000
Gross Unrealized Losses	(196,145,000)	(140,657,000)
Fair Value	27,482,932,000	27,214,682,000
Total OTTI Recognized in OCI	5,921,000	(6,135,000)
Amortized cost and fair value of the Company's investments classified as held-to-maturity
Fair Value	334,579,000	319,163,000
Amortized Cost
Due in one year or less	462,439,000
Due after one year through five years	4,474,623,000
Due after five years through ten years	6,601,844,000
Due after ten years	12,689,719,000
Total	24,228,625,000
Fair Value
Due in one year or less	469,358,000
Due after one year through five years	4,930,066,000
Due after five years through ten years	7,235,764,000
Due after ten years	14,444,373,000
Total	27,079,561,000
Amortized Cost
Due after ten years	315,000,000
Total	315,000,000
Fair Value
Due after ten years	334,579,000
Total	334,579,000
Fixed maturities:
Amortized cost and fair value of the Company's investments classified as available-for-sale
Amortized Cost	24,228,625,000	23,654,002,000
Gross Unrealized Gains	3,040,358,000	3,239,332,000
Gross Unrealized Losses	(189,422,000)	(130,664,000)
Fair Value	27,079,561,000	26,762,670,000
Total OTTI Recognized in OCI	6,164,000	(6,135,000)
Amortized cost and fair value of the Company's investments classified as held-to-maturity
Amortized Cost	315,000,000	300,000,000
Gross Unrealized Gains	19,579,000	19,163,000
Fair Value	334,579,000	319,163,000
Trading securities	3,000,000,000	3,000,000,000
Residential mortgage-backed securities
Amortized cost and fair value of the Company's investments classified as available-for-sale
Amortized Cost	1,665,681,000	1,766,260,000
Gross Unrealized Gains	89,005,000	92,417,000
Gross Unrealized Losses	(36,776,000)	(19,347,000)
Fair Value	1,717,910,000	1,839,330,000
Total OTTI Recognized in OCI	6,353,000	(406,000)
Commercial mortgage-backed securities
Amortized cost and fair value of the Company's investments classified as available-for-sale
Amortized Cost	853,334,000	797,844,000
Gross Unrealized Gains	64,906,000	72,577,000
Gross Unrealized Losses	(2,085,000)	(598,000)
Fair Value	916,155,000	869,823,000
Other asset-backed securities
Amortized cost and fair value of the Company's investments classified as available-for-sale
Amortized Cost	1,013,575,000	1,023,649,000
Gross Unrealized Gains	17,325,000	12,788,000
Gross Unrealized Losses	(84,339,000)	(61,424,000)
Fair Value	946,561,000	975,013,000
Total OTTI Recognized in OCI	(189,000)	(241,000)
U.S. government-related securities
Amortized cost and fair value of the Company's investments classified as available-for-sale
Amortized Cost	1,142,772,000	1,097,501,000
Gross Unrealized Gains	66,003,000	71,536,000
Gross Unrealized Losses	(1,723,000)	(591,000)
Fair Value	1,207,052,000	1,168,446,000
Other government-related securities
Amortized cost and fair value of the Company's investments classified as available-for-sale
Amortized Cost	82,172,000	93,565,000
Gross Unrealized Gains	6,323,000	7,258,000
Gross Unrealized Losses	(18,000)	(45,000)
Fair Value	88,477,000	100,778,000
States, municipals, and political subdivisions
Amortized cost and fair value of the Company's investments classified as available-for-sale
Amortized Cost	1,179,314,000	1,188,019,000
Gross Unrealized Gains	254,476,000	255,898,000
Gross Unrealized Losses	(269,000)	(264,000)
Fair Value	1,433,521,000	1,443,653,000
Corporate bonds
Amortized cost and fair value of the Company's investments classified as available-for-sale
Amortized Cost	18,291,777,000	17,687,164,000
Gross Unrealized Gains	2,542,320,000	2,726,858,000
Gross Unrealized Losses	(64,212,000)	(48,395,000)
Fair Value	20,769,885,000	20,365,627,000
Total OTTI Recognized in OCI		(5,488,000)
Other
Amortized cost and fair value of the Company's investments classified as held-to-maturity
Amortized Cost	315,000,000	300,000,000
Gross Unrealized Gains	19,579,000	19,163,000
Fair Value	334,579,000	319,163,000
Equity securities
Amortized cost and fair value of the Company's investments classified as available-for-sale
Amortized Cost	345,963,000	352,272,000
Gross Unrealized Gains	14,440,000	11,881,000
Gross Unrealized Losses	(6,723,000)	(9,993,000)
Fair Value	353,680,000	354,160,000
Total OTTI Recognized in OCI	(243,000)
Amortized cost and fair value of the Company's investments classified as held-to-maturity
Trading securities	22,400,000	19,600,000
Short-term investments
Amortized cost and fair value of the Company's investments classified as available-for-sale
Amortized Cost	49,691,000	97,852,000
Fair Value	49,691,000	97,852,000
Amortized cost and fair value of the Company's investments classified as held-to-maturity
Trading securities	$ 110,600,000	$ 118,900,000