Investments (Details 6) (USD $)	Mar. 31, 2012	Dec. 31, 2011
Corporate Secturities by Investment Sector And Credit Rating [Line Items]
Available For Sale Securities Amortized Cost	$ 7,518,983,000	$ 6,931,958,000
Available For Sale Securities Fair Value Disclosure	8,111,454,000	7,461,106,000
Percentage Of Total Available For Sale Securities	100.00%	100.00%
Finance Industry [Member]
Corporate Secturities by Investment Sector And Credit Rating [Line Items]
Available For Sale Securities Amortized Cost	2,578,694,000	2,411,175,000
Available For Sale Securities Fair Value Disclosure	2,686,141,000	2,442,149,000
Percentage Of Total Available For Sale Securities	33.10%	32.70%
Industrial [Member]
Corporate Secturities by Investment Sector And Credit Rating [Line Items]
Available For Sale Securities Amortized Cost	3,751,858,000	3,402,099,000
Available For Sale Securities Fair Value Disclosure	4,109,403,000	3,760,187,000
Percentage Of Total Available For Sale Securities	50.70%	50.40%
Utility Industry [Member]
Corporate Secturities by Investment Sector And Credit Rating [Line Items]
Available For Sale Securities Amortized Cost	1,179,592,000	1,115,384,000
Available For Sale Securities Fair Value Disclosure	1,306,787,000	1,255,090,000
Percentage Of Total Available For Sale Securities	16.10%	16.90%
Other Industry [Member]
Corporate Secturities by Investment Sector And Credit Rating [Line Items]
Available For Sale Securities Amortized Cost	8,839,000	3,300,000
Available For Sale Securities Fair Value Disclosure	$ 9,123,000	$ 3,680,000
Percentage Of Total Available For Sale Securities	0.10%	0.00%

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

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