Guarantee of Securities of Subsidiaries Balance Sheets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Condensed Financial Statements, Captions [Line Items]
Subsidiary, Cumulative Percentage Ownership after All Transactions	100.00%
Equity Restrictions	$ 0.0	$ 0.0
Other Restricted Assets	$ 0.0	0.0
Restriction to Transfer Funds	Boardwalk Pipelines (Subsidiary Issuer) has issued securities which have been fully and unconditionally guaranteed by the Company (Parent Guarantor). The Subsidiary Issuer is 100% owned by the Parent Guarantor. The Company's subsidiaries had no significant restrictions on their ability to pay distributions or make loans to the Company except as noted in their debt covenants and had no restricted assets as of December 31, 2019 and 2018.
Assets
Cash and cash equivalents	$ 3.7	3.6	$ 17.6	$ 4.6
Receivables	132.4	153.7
Receivables - affiliate	0.0	0.0
Costs recoverable from customers	4.4	23.6
Prepayments	16.0	21.3
Advances to affiliates	0.0	0.0
Other current assets	11.2	10.1
Total current assets	167.7	212.3
Investment in consolidated subsidiaries	0.0	0.0
Property, plant and equipment, gross	11,743.4	11,325.6
Less—accumulated depreciation and amortization	3,263.7	2,939.8
Property, plant and equipment, net	8,479.7	8,385.8
Advances to affiliates – noncurrent	0.0	0.0
Other noncurrent assets	495.7	450.6
Total other assets	495.7	450.6
Total Assets	9,143.1	9,048.7
Liabilities and Partners' Capital
Payables	77.4	71.1
Payable to affiliates	0.5	0.5
Advances from affiliates	4.1	0.0
Other current liabilities	187.5	185.8
Total current liabilities	269.5	257.4
Long-term debt and finance lease obligation	3,566.1	3,701.3
Advances from affiliates - noncurrent	0.0	0.0
Other noncurrent liabilities	248.0	228.1
Total other liabilities and deferred credits	248.0	228.1
Total partners’ capital	5,059.5	4,861.9
Total Liabilities and Partners' Capital	9,143.1	9,048.7
Reportable Legal Entities | Parent Guarantor
Assets
Cash and cash equivalents	0.0	0.3	0.3	0.6
Receivables	0.0	0.0
Receivables - affiliate	0.0	0.0
Costs recoverable from customers	0.0	0.0
Prepayments	0.3	0.3
Advances to affiliates	0.0	0.0
Other current assets	0.0	0.0
Total current assets	0.3	0.6
Investment in consolidated subsidiaries	3,059.4	2,828.1
Property, plant and equipment, gross	0.6	0.6
Less—accumulated depreciation and amortization	0.6	0.6
Property, plant and equipment, net	0.0	0.0
Advances to affiliates – noncurrent	2,004.9	2,034.2
Other noncurrent assets	0.0	0.2
Total other assets	2,004.9	2,034.4
Total Assets	5,064.6	4,863.1
Liabilities and Partners' Capital
Payables	0.5	0.6
Payable to affiliates	0.5	0.5
Advances from affiliates	4.1	0.0
Other current liabilities	0.0	0.1
Total current liabilities	5.1	1.2
Long-term debt and finance lease obligation	0.0	0.0
Advances from affiliates - noncurrent	0.0	0.0
Other noncurrent liabilities	0.0	0.0
Total other liabilities and deferred credits	0.0	0.0
Total partners’ capital	5,059.5	4,861.9
Total Liabilities and Partners' Capital	5,064.6	4,863.1
Reportable Legal Entities | Subsidiary Issuer
Assets
Cash and cash equivalents	2.1	1.6	4.6	1.8
Receivables	0.0	0.0
Receivables - affiliate	0.0	0.0
Costs recoverable from customers	0.0	0.0
Prepayments	0.0	0.0
Advances to affiliates	33.7	0.0
Other current assets	0.0	0.0
Total current assets	35.8	1.6
Investment in consolidated subsidiaries	7,230.5	7,136.6
Property, plant and equipment, gross	0.0	0.0
Less—accumulated depreciation and amortization	0.0	0.0
Property, plant and equipment, net	0.0	0.0
Advances to affiliates – noncurrent	377.1	460.1
Other noncurrent assets	3.8	2.5
Total other assets	380.9	462.6
Total Assets	7,647.2	7,600.8
Liabilities and Partners' Capital
Payables	0.1	0.1
Payable to affiliates	0.0	0.0
Advances from affiliates	1.6	2.0
Other current liabilities	23.0	24.3
Total current liabilities	24.7	26.4
Long-term debt and finance lease obligation	2,428.7	2,280.1
Advances from affiliates - noncurrent	2,132.7	2,466.0
Other noncurrent liabilities	1.7	0.2
Total other liabilities and deferred credits	2,134.4	2,466.2
Total partners’ capital	3,059.4	2,828.1
Total Liabilities and Partners' Capital	7,647.2	7,600.8
Reportable Legal Entities | Non-Guarantor Subsidiaries
Assets
Cash and cash equivalents	1.6	1.7	12.7	2.2
Receivables	132.4	153.7
Receivables - affiliate	7.0	9.5
Costs recoverable from customers	4.4	23.6
Prepayments	15.7	21.0
Advances to affiliates	1.6	2.0
Other current assets	15.6	14.3
Total current assets	178.3	225.8
Investment in consolidated subsidiaries	0.0	0.0
Property, plant and equipment, gross	11,742.8	11,325.0
Less—accumulated depreciation and amortization	3,263.1	2,939.2
Property, plant and equipment, net	8,479.7	8,385.8
Advances to affiliates – noncurrent	127.8	431.8
Other noncurrent assets	491.0	446.5
Total other assets	618.8	878.3
Total Assets	9,276.8	9,489.9
Liabilities and Partners' Capital
Payables	76.8	70.4
Payable to affiliates	7.0	9.5
Advances from affiliates	33.7	0.0
Other current liabilities	168.3	164.2
Total current liabilities	285.8	244.1
Long-term debt and finance lease obligation	1,137.4	1,421.2
Advances from affiliates - noncurrent	377.1	460.1
Other noncurrent liabilities	246.0	227.9
Total other liabilities and deferred credits	623.1	688.0
Total partners’ capital	7,230.5	7,136.6
Total Liabilities and Partners' Capital	9,276.8	9,489.9
Eliminations
Assets
Cash and cash equivalents	0.0	0.0	$ 0.0	$ 0.0
Receivables	0.0	0.0
Receivables - affiliate	(7.0)	(9.5)
Costs recoverable from customers	0.0	0.0
Prepayments	0.0	0.0
Advances to affiliates	(35.3)	(2.0)
Other current assets	(4.4)	(4.2)
Total current assets	(46.7)	(15.7)
Investment in consolidated subsidiaries	(10,289.9)	(9,964.7)
Property, plant and equipment, gross	0.0	0.0
Less—accumulated depreciation and amortization	0.0	0.0
Property, plant and equipment, net	0.0	0.0
Advances to affiliates – noncurrent	(2,509.8)	(2,926.1)
Other noncurrent assets	0.9	1.4
Total other assets	(2,508.9)	(2,924.7)
Total Assets	(12,845.5)	(12,905.1)
Liabilities and Partners' Capital
Payables	0.0	0.0
Payable to affiliates	(7.0)	(9.5)
Advances from affiliates	(35.3)	(2.0)
Other current liabilities	(3.8)	(2.8)
Total current liabilities	(46.1)	(14.3)
Long-term debt and finance lease obligation	0.0	0.0
Advances from affiliates - noncurrent	(2,509.8)	(2,926.1)
Other noncurrent liabilities	0.3	0.0
Total other liabilities and deferred credits	(2,509.5)	(2,926.1)
Total partners’ capital	(10,289.9)	(9,964.7)
Total Liabilities and Partners' Capital	$ (12,845.5)	$ (12,905.1)