INVESTMENT OPERATIONS (Details) (USD $)	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Major categories of net investment income
Fixed maturities									$ 1,453,702,000	$ 1,416,861,000	$ 1,302,226,000
Equity securities									21,187,000	21,274,000	18,516,000
Mortgage loans									349,877,000	336,542,000	311,253,000
Investment real estate									3,290,000	3,459,000	3,180,000
Short-term investments									64,729,000	69,791,000	72,803,000
Investment income before other investment expenses									1,892,785,000	1,847,927,000	1,707,978,000
Other investment expenses									30,453,000	27,284,000	24,302,000
Net investment income	476,045,000	467,944,000	456,222,000	462,121,000	464,719,000	462,926,000	448,785,000	444,213,000	1,862,332,000	1,820,643,000	1,683,676,000
Net realized investment gains (losses) for all other investments
Fixed maturities									67,726,000	80,180,000	50,056,000
Equity securities									(45,000)	9,194,000	6,488,000
Impairments on fixed maturity securities									(58,886,000)	(47,442,000)	(39,696,000)
Impairments on equity securities											(1,814,000)
Modco trading portfolio									177,986,000	164,224,000	109,399,000
Other investments									(14,632,000)	(18,683,000)	(11,577,000)
Total realized gains (losses) - investments									172,149,000	187,473,000	112,856,000
Gross realized gains on investments available-for-sale (fixed maturities, equity securities, and short-term investments)									73,300,000	104,800,000	98,200,000
Gross realized losses									64,000,000	62,100,000	82,900,000
Impairment losses on investments available-for-sale									58,400,000	46,700,000	41,300,000
Fair value (proceeds) of securities in an unrealized gain position sold									1,600,000,000	2,200,000,000	2,900,000,000
Gain realized on the sale of securities in an unrealized gain position									73,300,000	104,800,000	98,200,000
Fair value (proceeds) of securities in an unrealized loss position sold									38,000,000	264,000,000	705,500,000
Loss realized on the sale of securities in an unrealized loss position									$ 5,600,000	$ 15,400,000	$ 41,600,000