Financing - Debt (Details) - USD ($) $ in Millions				9 Months Ended		12 Months Ended
		Aug. 31, 2020	May 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Oct. 30, 2020
Debt Instrument [Line Items]
Long-term debt and finance lease obligation				$ 3,769.6		$ 3,566.1
Repayment of borrowings from long-term debt				0.0	$ 350.0
Unsecured Debt
Debt Instrument [Line Items]
Long-term Debt, Gross				$ 3,800.0		$ 3,300.0
Debt, Weighted-Average Interest Rate				4.86%		5.06%
Debt Instrument, Covenant Compliance				As of September 30, 2020, Boardwalk Pipelines and its operating subsidiaries were in compliance with their debt covenants.
Short-term Debt, Refinanced, Description				The Company has included the $440.0 million aggregate principal amount of Texas Gas 4.50% notes due 2021 (Texas Gas 2021 Notes) which mature in less than one year as long-term debt on its Condensed Consolidated Balance Sheets as of September 30, 2020, since the Company has the intent and the ability to refinance the notes through the available borrowing capacity under its revolving credit facility. In September 2020, the Company submitted its notice of redemption to retire the Texas Gas 2021 Notes on November 3, 2020, at a redemption price of 100% of the principal amount of such Notes, plus any unpaid and accrued interest. The Company will use its available cash and borrowings under the revolving credit facility to retire the Texas Gas 2021 Notes, with a portion of the available cash coming from the proceeds from the August 2020 issuance of $500.0 million aggregate principal amount of Boardwalk Pipelines 3.40% notes due 2031 discussed below.
Unsecured Debt | Texas Gas Notes Due 2021
Debt Instrument [Line Items]
Short-term Debt, Refinanced, Amount				$ 440.0
Debt Instrument, Interest Rate, Stated Percentage				4.50%
Debt Instrument, Redemption Price, Percentage				100.00%
Unsecured Debt | Boardwalk Pipelines 3.40% Notes Due 2031
Debt Instrument [Line Items]
Debt Instrument, Issuance Date		Aug. 31, 2020
Debt Instrument, Face Amount		$ 500.0
Payments of Debt Issuance Costs		5.0
Proceeds from Debt, Net of Issuance Costs	[1]	$ 495.0
Debt Instrument, Interest Rate, Stated Percentage		3.40%
Debt Instrument, Maturity Date		Feb. 15, 2031
Unsecured Debt | Boardwalk Pipelines 4.8% Notes Due 2029
Debt Instrument [Line Items]
Debt Instrument, Issuance Date			May 31, 2019
Debt Instrument, Face Amount			$ 500.0
Payments of Debt Issuance Costs			4.8
Proceeds from Debt, Net of Issuance Costs	[2]		$ 495.2
Debt Instrument, Interest Rate, Stated Percentage			4.80%
Debt Instrument, Maturity Date			May 03, 2029
Unsecured Debt | Boardwalk Pipelines Notes Due 2019
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage						5.75%
Repayment of borrowings from long-term debt						$ 350.0
Line of Credit
Debt Instrument [Line Items]
Debt, Weighted-Average Interest Rate						3.00%
Line of Credit Facility [Abstract]
Long-term Line of Credit				$ 0.0		$ 295.0
Line of Credit Facility, Covenant Compliance				The Company and its subsidiaries were in compliance with all covenant requirements under the revolving credit facility as of September 30, 2020.
Line of Credit | Subsequent Event
Line of Credit Facility [Abstract]
Long-term Line of Credit							$ 80.0
Line of Credit Facility, Remaining Borrowing Capacity							$ 1,400.0
Line of Credit | Amendment No. 2 Credit Agreement - 2017
Line of Credit Facility [Abstract]
Line of Credit Facility, Maximum Borrowing Capacity				$ 1,475.0
Line of Credit Facility, Expiration Date				May 26, 2022

[1] The net proceeds of this offering will be used to retire the Texas Gas 2021 Notes on November 3, 2020, to fund growth capital expenditures and for general partnership purposes. Initially, the Company used the net proceeds to reduce outstanding borrowings under its revolving credit facility.

[2] The net proceeds of this offering were used to retire the outstanding $350.0 million aggregate principal amount of Boardwalk Pipelines 5.75% notes due 2019 (Boardwalk Pipelines 2019 Notes) at maturity and for general partnership purposes. Initially, the Company used the net proceeds to reduce outstanding borrowings under its revolving credit facility. Subsequently, in September 2019, the Company retired all of the outstanding aggregate principal amount of Boardwalk Pipelines 2019 Notes at maturity with borrowings under its revolving credit facility.