Operating Segment - Narrative (Details) - Transaction costs associated with Dominion Questar combination - Dominion Questar Corporation - Dominion Questar Corporation - USD ($)		3 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Transaction costs		$ 300,000	$ 0
Corporate and Other | Operating Segments
Segment Reporting Information [Line Items]
Transaction costs	[1]	300,000
Transaction costs, net of tax		$ 200,000

[1] See Note 11 for amounts attributable to related parties.