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