Fair Value Measurements - Schedule of Carrying Amounts and Estimated Fair Values of Financial Instruments not Recorded at Fair Value (Details) - USD ($) $ in Millions		Sep. 30, 2016		Dec. 31, 2015
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	[1]	$ 531.4		$ 531.2
Estimated Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt	[1]	$ 607.2	[2]	$ 568.4	[3]

[1] Carrying amount includes amounts which represent the unamortized debt issuance costs, discount and/or premium.

[2] Fair value is estimated using market prices, where available, and interest rates currently available for issuance of debt with similar terms and remaining maturities. The fair value measurements are classified as Level 2.

[3] Fair value is estimated using the discounted present value of cash flows using Questar Gas’ current credit risk-adjusted borrowing rates. The fair value measurements are classified as Level 2.