DERIVATIVES AND HEDGE ACCOUNTING ACTIVITIES - Schedule of Fair Values of Derivatives and Balance Sheet Location (Details) - USD ($) $ in Millions		Dec. 31, 2017	Dec. 31, 2016
Derivatives Fair Value [Line Items]
Derivatives assets		$ 2.4	$ 0.1
Derivative liabilities			0.1
Current Assets
Derivatives Fair Value [Line Items]
Derivatives assets	[1]	0.4	0.1
Noncurrent Assets
Derivatives Fair Value [Line Items]
Derivatives assets	[2]	2.0
Current Liabilities
Derivatives Fair Value [Line Items]
Derivative liabilities	[3]		0.1
Commodity | Current Assets
Derivatives Fair Value [Line Items]
Derivatives assets		0.4	0.1
Commodity | Noncurrent Assets
Derivatives Fair Value [Line Items]
Derivatives assets		2.0
Commodity | Current Liabilities
Derivatives Fair Value [Line Items]
Derivative liabilities			0.1
Fair Value - Derivatives not under Hedge Accounting
Derivatives Fair Value [Line Items]
Derivatives assets		2.4	0.1
Derivative liabilities			0.1
Fair Value - Derivatives not under Hedge Accounting | Current Assets
Derivatives Fair Value [Line Items]
Derivatives assets	[1]	0.4	0.1
Fair Value - Derivatives not under Hedge Accounting | Noncurrent Assets
Derivatives Fair Value [Line Items]
Derivatives assets	[2]	2.0
Fair Value - Derivatives not under Hedge Accounting | Current Liabilities
Derivatives Fair Value [Line Items]
Derivative liabilities	[3]		0.1
Fair Value - Derivatives not under Hedge Accounting | Commodity | Current Assets
Derivatives Fair Value [Line Items]
Derivatives assets		0.4	0.1
Fair Value - Derivatives not under Hedge Accounting | Commodity | Noncurrent Assets
Derivatives Fair Value [Line Items]
Derivatives assets		$ 2.0
Fair Value - Derivatives not under Hedge Accounting | Commodity | Current Liabilities
Derivatives Fair Value [Line Items]
Derivative liabilities			$ 0.1

[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.

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