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Invesments (Aggregated Summarized Financial Information for Partnerhips in our Alternative Investment Portfolio) (Details) (USD $)	3 Months Ended				9 Months Ended
	Sep. 30, 2011	Jun. 30, 2011	Sep. 30, 2010	Jun. 30, 2010	Sep. 30, 2011	Jun. 30, 2011	Sep. 30, 2010	Jun. 30, 2010
Net investment income		$ 136,800,000		$ 119,900,000		$ 423,600,000		$ 394,300,000
Realized gains (losses)		710,600,000		75,000,000		873,600,000		(411,600,000)
Net change in unrealized appreciation		(194,700,000)		129,600,000		1,877,800,000		1,378,300,000
Net income		652,700,000		324,500,000		3,175,000,000		1,361,000,000
Net investment income earned	35,786,000		32,986,000		118,604,000		104,237,000
Alternative Investments [Member]
Net investment income earned	$ 4,500,000		$ 2,300,000		$ 24,000,000		$ 11,100,000

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