Operating Revenue - Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	[1]	Mar. 31, 2017
Regulated And Unregulated Operating Revenue [Abstract]
Contract liability balances	$ 9.9	$ 17.3		$ 17.3
Contract liability, revenue recognized	$ 17.3

[1] Questar Gas’ Balance Sheet at December 31, 2017 has been derived from the audited Balance Sheet at that date.