Derivatives and Hedge Accounting Activities - Schedule of Fair Values of Derivatives and Balance Sheet Location (Details) - USD ($) $ in Millions		Jun. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Derivatives assets		$ 3.9	$ 2.4
Current Assets
Derivatives, Fair Value [Line Items]
Derivatives assets	[1]	1.2	0.4
Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivatives assets	[2]	2.7	2.0
Commodity | Current Assets
Derivatives, Fair Value [Line Items]
Derivatives assets		1.2	0.4
Commodity | Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivatives assets		2.7	2.0
Fair Value - Derivatives not under Hedge Accounting
Derivatives, Fair Value [Line Items]
Derivatives assets		3.9	2.4
Fair Value - Derivatives not under Hedge Accounting | Current Assets
Derivatives, Fair Value [Line Items]
Derivatives assets	[1]	1.2	0.4
Fair Value - Derivatives not under Hedge Accounting | Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivatives assets	[2]	2.7	2.0
Fair Value - Derivatives not under Hedge Accounting | Commodity | Current Assets
Derivatives, Fair Value [Line Items]
Derivatives assets		1.2	0.4
Fair Value - Derivatives not under Hedge Accounting | Commodity | Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivatives assets		$ 2.7	$ 2.0

[1] Current derivative assets are presented in other current assets in Questar Gas’Balance Sheets.

[2] Noncurrent derivative assets are presented in other deferred charges and other assets in Questar Gas’ Balance Sheets.