Guarantee of Securities of Subsidiaries Balance Sheets (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
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 Partnership (Parent Guarantor). The Subsidiary Issuer is 100% owned by the Parent Guarantor. The Partnership's subsidiaries had no significant restrictions on their ability to pay distributions or make loans to the Partnership except as noted in the debt covenants and had no restricted assets as of March 31, 2019, and December 31, 2018.
Assets
Cash and cash equivalents	$ 15.3	3.6	$ 33.4	$ 17.6
Receivables	133.8	153.7
Receivables - affiliate	0.0	0.0
Costs recoverable from customers	16.2	23.6
Prepayments	13.6	21.3
Advances to affiliates	0.0	0.0
Other current assets	7.7	10.1
Total current assets	186.6	212.3
Investment in consolidated subsidiaries	0.0	0.0
Property, plant and equipment, gross	11,390.3	11,325.6
Less—accumulated depreciation and amortization	3,015.9	2,939.8
Property, plant and equipment, net	8,374.4	8,385.8
Advances to affiliates - noncurrent	0.0	0.0
Other noncurrent assets	475.1	450.6
Total other assets	475.1	450.6
Total Assets	9,036.1	9,048.7
Liabilities and Partners' Capital
Payables	58.1	71.1
Payable to affiliates	0.5	0.5
Advances from affiliates	0.0	0.0
Other current liabilities	142.7	185.8
Total current liabilities	201.3	257.4
Long-term debt and finance lease obligation	3,647.4	3,701.3
Advances from affiliates - noncurrent	0.0	0.0
Other noncurrent liabilities	242.2	228.1
Total other liabilities and deferred credits	242.2	228.1
Total partners' capital	4,945.2	4,861.9
Total Liabilities and Partners' Capital	9,036.1	9,048.7
Reportable Legal Entities | Parent Guarantor
Assets
Cash and cash equivalents	0.1	0.3	0.3	0.3
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	0.4	0.6
Investment in consolidated subsidiaries	2,918.1	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	2,027.6	2,034.2
Other noncurrent assets	0.0	0.2
Total other assets	2,027.6	2,034.4
Total Assets	4,946.1	4,863.1
Liabilities and Partners' Capital
Payables	0.4	0.6
Payable to affiliates	0.5	0.5
Advances from affiliates	0.0	0.0
Other current liabilities	0.0	0.1
Total current liabilities	0.9	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	4,945.2	4,861.9
Total Liabilities and Partners' Capital	4,946.1	4,863.1
Reportable Legal Entities | Subsidiary Issuer
Assets
Cash and cash equivalents	2.4	1.6	18.8	4.6
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	0.0	0.0
Other current assets	0.0	0.0
Total current assets	2.4	1.6
Investment in consolidated subsidiaries	7,195.2	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	490.3	460.1
Other noncurrent assets	5.2	2.5
Total other assets	495.5	462.6
Total Assets	7,693.1	7,600.8
Liabilities and Partners' Capital
Payables	0.1	0.1
Payable to affiliates	0.0	0.0
Advances from affiliates	2.2	2.0
Other current liabilities	27.4	24.3
Total current liabilities	29.7	26.4
Long-term debt and finance lease obligation	2,280.9	2,280.1
Advances from affiliates - noncurrent	2,461.7	2,466.0
Other noncurrent liabilities	2.7	0.2
Total other liabilities and deferred credits	2,464.4	2,466.2
Total partners' capital	2,918.1	2,828.1
Total Liabilities and Partners' Capital	7,693.1	7,600.8
Reportable Legal Entities | Non-Guarantor Subsidiaries
Assets
Cash and cash equivalents	12.8	1.7	14.3	12.7
Receivables	133.8	153.7
Receivables - affiliate	7.1	9.5
Costs recoverable from customers	16.2	23.6
Prepayments	13.3	21.0
Advances to affiliates	2.2	2.0
Other current assets	9.9	14.3
Total current assets	195.3	225.8
Investment in consolidated subsidiaries	0.0	0.0
Property, plant and equipment, gross	11,389.7	11,325.0
Less—accumulated depreciation and amortization	3,015.3	2,939.2
Property, plant and equipment, net	8,374.4	8,385.8
Advances to affiliates - noncurrent	434.1	431.8
Other noncurrent assets	469.7	446.5
Total other assets	903.8	878.3
Total Assets	9,473.5	9,489.9
Liabilities and Partners' Capital
Payables	57.6	70.4
Payable to affiliates	7.1	9.5
Advances from affiliates	0.0	0.0
Other current liabilities	117.2	164.2
Total current liabilities	181.9	244.1
Long-term debt and finance lease obligation	1,366.5	1,421.2
Advances from affiliates - noncurrent	490.3	460.1
Other noncurrent liabilities	239.6	227.9
Total other liabilities and deferred credits	729.9	688.0
Total partners' capital	7,195.2	7,136.6
Total Liabilities and Partners' Capital	9,473.5	9,489.9
Eliminations
Assets
Cash and cash equivalents	0.0	0.0	$ 0.0	$ 0.0
Receivables	0.0	0.0
Receivables - affiliate	(7.1)	(9.5)
Costs recoverable from customers	0.0	0.0
Prepayments	0.0	0.0
Advances to affiliates	(2.2)	(2.0)
Other current assets	(2.2)	(4.2)
Total current assets	(11.5)	(15.7)
Investment in consolidated subsidiaries	(10,113.3)	(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	(2,952.0)	(2,926.1)
Other noncurrent assets	0.2	1.4
Total other assets	(2,951.8)	(2,924.7)
Total Assets	(13,076.6)	(12,905.1)
Liabilities and Partners' Capital
Payables	0.0	0.0
Payable to affiliates	(7.1)	(9.5)
Advances from affiliates	(2.2)	(2.0)
Other current liabilities	(1.9)	(2.8)
Total current liabilities	(11.2)	(14.3)
Long-term debt and finance lease obligation	0.0	0.0
Advances from affiliates - noncurrent	(2,952.0)	(2,926.1)
Other noncurrent liabilities	(0.1)	0.0
Total other liabilities and deferred credits	(2,952.1)	(2,926.1)
Total partners' capital	(10,113.3)	(9,964.7)
Total Liabilities and Partners' Capital	$ (13,076.6)	$ (12,905.1)