Other Material Contingencies Other Material Contingencies - Other (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Guarantor Obligations [Line Items]
Guarantee obligations maximum exposure	$ 2,128	$ 2,073
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

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 460
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=82913245&loc=d3e12069-110248

Name:	us-gaap_GuaranteeObligationsMaximumExposure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant