Segment and Related Information - Gas Revenues (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Gas revenue	$ 3,920	$ 1,652
Gas Distribution Operations
Segment Reporting Information [Line Items]
Gas revenue	3,024	1,266
Gas Marketing Services
Segment Reporting Information [Line Items]
Gas revenue	860	354
Other Gas Revenue
Segment Reporting Information [Line Items]
Gas revenue	$ 36	$ 32