Derivatives and Hedge Accounting Activities - Schedule of Fair Values of Derivatives and Balance Sheet Location (Details) - USD ($) $ in Millions		Sep. 30, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Derivatives assets		$ 0.7	$ 0.1
Derivative liabilities		0.5	0.1
Current Assets
Derivatives, Fair Value [Line Items]
Derivatives assets	[1]	0.6	0.1
Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	[2]	0.5	0.1
Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivatives assets	[3]	0.1
Commodity | Current Assets
Derivatives, Fair Value [Line Items]
Derivatives assets		0.6	0.1
Commodity | Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities		0.5	0.1
Commodity | Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivatives assets		0.1
Fair Value - Derivatives not under Hedge Accounting
Derivatives, Fair Value [Line Items]
Derivatives assets		0.7	0.1
Derivative liabilities		0.5	0.1
Fair Value - Derivatives not under Hedge Accounting | Current Assets
Derivatives, Fair Value [Line Items]
Derivatives assets	[1]	0.6	0.1
Fair Value - Derivatives not under Hedge Accounting | Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	[2]	0.5	0.1
Fair Value - Derivatives not under Hedge Accounting | Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivatives assets	[3]	0.1
Fair Value - Derivatives not under Hedge Accounting | Commodity | Current Assets
Derivatives, Fair Value [Line Items]
Derivatives assets		0.6	0.1
Fair Value - Derivatives not under Hedge Accounting | Commodity | Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities		0.5	$ 0.1
Fair Value - Derivatives not under Hedge Accounting | Commodity | Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivatives assets		$ 0.1

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

[2] Current derivative liabilities are presented in other current liabilities in Questar Gas’ Balance Sheets.

[3] Noncurrent derivative assets are presented in other noncurrent assets in Questar Gas' Balance Sheets.