Summary of Significant Accounting Policies (Sale of SoCore Energy) (Details) - SoCore Energy $ in Millions	6 Months Ended
Jun. 30, 2018 USD ($)
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Pre-tax loss	$ 63
After-tax loss	$ 46

Amount after tax of gain (loss) not previously recognized resulting from the disposal of a discontinued operation.

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