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Acquisitions and Dispositions (Acquisitions Narrative) (Details) - USD ($) $ in Millions		3 Months Ended				12 Months Ended
	Oct. 03, 2016	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Pretax transaction and integration costs		$ 31	$ 16	$ 20	$ 17		$ 84
Impairment charges						$ (8)	402	$ 282
Piedmont
Business Acquisition [Line Items]
Total cash purchase price	$ 5,000
Long-term debt assumed	$ 2,000
Pretax transaction and integration costs							84	103
Impairment charges								7
Piedmont \| Operation, maintenance and other
Business Acquisition [Line Items]
Pretax transaction and integration costs								64
Piedmont \| System Integration Costs
Business Acquisition [Line Items]
Pretax transaction and integration costs							$ 78	$ 71