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									$ 1,453,018,000	$ 1,414,965,000		$ 1,301,047,000
Equity securities									20,740,000	20,595,000		17,836,000
Mortgage loans									349,845,000	336,541,000		310,988,000
Investment real estate									3,289,000	3,458,000		3,180,000
Short-term investments									62,887,000	72,137,000		77,185,000
Investment income before other investment expenses									1,889,779,000	1,847,696,000		1,710,236,000
Other investment expenses									100,441,000	94,252,000		85,391,000
Net investment income	460,784,000	446,374,000	438,648,000	443,532,000	445,780,000	445,928,000	434,425,000	427,311,000	1,789,338,000	1,753,444,000	[1]	1,624,845,000	[1]
Net realized investment gains (losses) for all other investments
Fixed maturities									67,669,000	80,044,000		51,816,000
Equity securities									(45,000)	9,136,000		6,489,000
Impairments on fixed maturity securities									(58,144,000)	(47,321,000)		(39,550,000)
Impairments on equity securities												(1,815,000)
Modco trading portfolio									177,986,000	164,224,000		109,399,000
Other investments									(12,774,000)	(5,651,000)		(9,283,000)
Total realized gains (losses) - investments									174,692,000	200,432,000		117,056,000
Gross realized gains on investments available-for-sale (fixed maturities, equity securities, and short-term investments)									73,200,000	104,500,000		99,800,000
Gross realized losses									60,300,000	62,000,000		82,600,000
Impairment losses on investments available-for-sale									54,700,000	46,600,000		41,100,000
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									73,200,000	104,500,000		99,800,000
Fair value (proceeds) of securities sold in an unrealized loss position									38,000,000	263,100,000		709,600,000
Loss realized on the sale of securities in an unrealized loss position									$ 5,600,000	$ 15,300,000		$ 41,500,000

[1] Recast from previously reported information