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Income Taxes (Details) (USD $) In Billions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Income Tax Disclosure [Abstract]
Deferred tax asset valuation allowance	$ 5.5	$ 4.8

The sum of the portions of deferred tax assets as of the balance sheet date for which, based on the weight of available evidence, it is more likely than not will not be realized through future reductions of tax-based income.

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