Financing - Debt (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Unsecured Debt
Debt Instrument [Line Items]
Short-term Debt, Refinanced, Amount	$ 350.0
Short-term Debt, Refinanced, Description	The Partnership has included $350.0 million of notes which mature in less than one year as long-term debt on its Condensed Consolidated Balance Sheets as of March 31, 2019. The Partnership has the intent and the ability to refinance the notes through the available borrowing capacity under its revolving credit facility as of March 31, 2019. The Partnership expects to retire these notes at their maturity.
Long-term Debt, Gross	$ 3,100.0	$ 3,100.0
Debt, Weighted-Average Interest Rate	5.17%	5.17%
Debt Instrument, Covenant Compliance	As of March 31, 2019, Boardwalk Pipelines and its operating subsidiaries were in compliance with their debt covenants.
Line of Credit
Debt Instrument [Line Items]
Debt, Weighted-Average Interest Rate	3.74%	3.69%
Line of Credit Facility [Abstract]
Long-term Line of Credit	$ 525.0	$ 580.0
Line of Credit Facility, Covenant Compliance	The Partnership and its subsidiaries were in compliance with all covenant requirements under the revolving credit facility as of March 31, 2019.
Line of Credit | Amendment No. 1 Credit Agreement - 2016
Line of Credit Facility [Abstract]
Line of Credit Facility, Maximum Borrowing Capacity	$ 1,500.0
Line of Credit | Amendment No. 2 Credit Agreement - 2017
Line of Credit Facility [Abstract]
Line of Credit Facility, Maximum Borrowing Capacity	$ 1,475.0