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Income Taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Income Tax Disclosure [Abstract]
Effective income tax rate (as a percent)	25.00%	(14.00%)
Production tax credits	$ 61	$ 79
Deferred income tax benefit associated with convertible investment tax credits	12	7
Consolidated effect of reversal of valuation allowance related to certain state investment tax credits		$ 26

- Definition

A ratio calculated by dividing the reported amount of income tax expense attributable to continuing operations for the period by GAAP-basis pretax income from continuing operations.

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- Definition

The amount of the change in the period in the valuation allowance for a specified deferred tax asset.

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