Derivatives and Hedge Accounting Activities - Schedule of Fair Values of Derivatives and Balance Sheet Location (Details) $ in Millions	Sep. 30, 2016 USD ($)
Derivatives, Fair Value [Line Items]
Derivatives assets	$ 1.6
Derivative liabilities	0.1
Current Assets
Derivatives, Fair Value [Line Items]
Derivatives assets	0.4	[1]
Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivatives assets	1.2	[2]
Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	0.1	[3]
Fair Value - Derivatives not under Hedge Accounting
Derivatives, Fair Value [Line Items]
Derivatives assets	1.6
Derivative liabilities	0.1
Fair Value - Derivatives not under Hedge Accounting | Current Assets
Derivatives, Fair Value [Line Items]
Derivatives assets	0.4	[1]
Fair Value - Derivatives not under Hedge Accounting | Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivatives assets	1.2	[2]
Fair Value - Derivatives not under Hedge Accounting | Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	0.1	[3]
Commodity | Current Assets
Derivatives, Fair Value [Line Items]
Derivatives assets	0.4
Commodity | Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivatives assets	1.2
Commodity | Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	0.1
Commodity | Fair Value - Derivatives not under Hedge Accounting | Current Assets
Derivatives, Fair Value [Line Items]
Derivatives assets	0.4
Commodity | Fair Value - Derivatives not under Hedge Accounting | Noncurrent Assets
Derivatives, Fair Value [Line Items]
Derivatives assets	1.2
Commodity | Fair Value - Derivatives not under Hedge Accounting | Current Liabilities
Derivatives, Fair Value [Line Items]
Derivative liabilities	$ 0.1

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

[2] Noncurrent derivative assets are presented in other noncurrent assets in Questar Gas’ Condensed Balance Sheets.

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