Other Material Contingencies Other Material Contingencies - Other (Details) - USD ($) $ in Millions	Sep. 30, 2017	Dec. 31, 2016
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	$ 2,162	$ 2,370
Financial Guarantee for Indemnity Agreements for Surety Bonds
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	70
Certain Obligations Under Agreement to Sell the Retail Electric Supply Business
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	53
Financial Guarantee to Public Utility Commission of Texas
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	$ 5