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, 2018

Dec. 31, 2017

Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]

Promissory Note Receivable from Dominion Energy

$ 1,986.0

Carrying Amount

Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]

Promissory Note Receivable from Dominion Energy

1,986.0

Long-term debt, including securities due within one year

^[1]

2,715.5

$ 730.7

Credit facility borrowings

73.0

Estimated Fair Value

Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]

Promissory Note Receivable from Dominion Energy

^[2]

1,997.1

Long-term debt, including securities due within one year

^[1],[2]

2,738.9

$ 760.7

Credit facility borrowings

^[2]

$ 73.0

[1]

Carrying amount includes amounts which represent the unamortized debt issuance costs, discount or premium.

[2]

Fair value is estimated using market prices, where available, and interest rates currently available for issuances of notes receivable and debt with similar terms and remaining maturities. All fair value measurements are classified as Level 2. The carrying amount of debt issues with short-term maturities and variable rates refinanced at current market rates is a reasonable estimate of their fair value.

Dominion Energy Midstream Partners, LP 10-Q