Guarantee of Securities of Subsidiaries Balance Sheets (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
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 at September 30, 2017, and December 31, 2016.
Assets
Cash and cash equivalents	$ 22.6	4.6	$ 111.4	$ 3.1
Receivables	108.2	139.8
Receivables - affiliate	0.0	0.0
Gas and liquids stored underground	5.6	1.3
Prepayments	19.8	17.7
Advances to affiliates	0.0	0.0
Other current assets	7.0	10.8
Total current assets	163.2	174.2
Investment in consolidated subsidiaries	0.0	0.0
Property, plant and equipment, gross	10,674.7	10,327.3
Less—accumulated depreciation and amortization	2,543.1	2,333.8
Property, plant and equipment, net	8,131.6	7,993.5
Advances to affiliates - noncurrent	0.0	0.0
Other noncurrent assets	467.3	470.1
Total other assets	467.3	470.1
Total Assets	8,762.1	8,637.8
Liabilities and Partners' Capital
Payables	87.9	137.5
Payable to affiliates	1.5	1.4
Advances from affiliates	0.0	0.0
Other current liabilities	203.3	194.0
Total current liabilities	292.7	332.9
Long-term debt and capital lease obligation	3,585.7	3,558.0
Payable to affiliate - noncurrent	16.0	16.0
Advances from affiliates - noncurrent	0.0	0.0
Other noncurrent liabilities	199.5	200.0
Total other liabilities and deferred credits	215.5	216.0
Total partners' capital	4,668.2	4,530.9
Total Liabilities and Partners' Capital	8,762.1	8,637.8
Reportable Legal Entities | Parent Guarantor
Assets
Cash and cash equivalents	0.4	0.6	0.3	0.0
Receivables	0.0	0.0
Receivables - affiliate	0.0	0.0
Gas and liquids stored underground	0.0	0.0
Prepayments	0.4	0.4
Advances to affiliates	0.0	0.0
Other current assets	0.0	0.0
Total current assets	0.8	1.0
Investment in consolidated subsidiaries	2,602.8	2,423.2
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	2,082.4	2,125.0
Other noncurrent assets	0.0	0.0
Total other assets	2,082.4	2,125.0
Total Assets	4,686.0	4,549.2
Liabilities and Partners' Capital
Payables	0.1	0.9
Payable to affiliates	1.5	1.4
Advances from affiliates	0.0	0.0
Other current liabilities	0.2	0.0
Total current liabilities	1.8	2.3
Long-term debt and capital lease obligation	0.0	0.0
Payable to affiliate - noncurrent	16.0	16.0
Advances from affiliates - noncurrent	0.0	0.0
Other noncurrent liabilities	0.0	0.0
Total other liabilities and deferred credits	16.0	16.0
Total partners' capital	4,668.2	4,530.9
Total Liabilities and Partners' Capital	4,686.0	4,549.2
Reportable Legal Entities | Subsidiary Issuer
Assets
Cash and cash equivalents	15.1	1.8	107.3	0.3
Receivables	0.0	0.0
Receivables - affiliate	0.0	0.0
Gas and liquids stored underground	0.0	0.0
Prepayments	0.0	0.0
Advances to affiliates	1.4	72.9
Other current assets	0.0	0.0
Total current assets	16.5	74.7
Investment in consolidated subsidiaries	6,665.8	6,653.6
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	886.4	435.0
Other noncurrent assets	3.4	3.3
Total other assets	889.8	438.3
Total Assets	7,572.1	7,166.6
Liabilities and Partners' Capital
Payables	0.2	0.2
Payable to affiliates	0.0	0.0
Advances from affiliates	22.4	102.7
Other current liabilities	29.9	21.8
Total current liabilities	52.5	124.7
Long-term debt and capital lease obligation	2,461.0	2,264.4
Payable to affiliate - noncurrent	0.0	0.0
Advances from affiliates - noncurrent	2,455.8	2,354.3
Other noncurrent liabilities	0.0	0.0
Total other liabilities and deferred credits	2,455.8	2,354.3
Total partners' capital	2,602.8	2,423.2
Total Liabilities and Partners' Capital	7,572.1	7,166.6
Reportable Legal Entities | Non-Guarantor Subsidiaries
Assets
Cash and cash equivalents	7.1	2.2	3.8	2.8
Receivables	108.2	139.8
Receivables - affiliate	6.9	7.0
Gas and liquids stored underground	5.6	1.3
Prepayments	19.4	17.3
Advances to affiliates	22.4	102.7
Other current assets	8.2	13.9
Total current assets	177.8	284.2
Investment in consolidated subsidiaries	0.0	0.0
Property, plant and equipment, gross	10,674.1	10,326.7
Less—accumulated depreciation and amortization	2,542.5	2,333.2
Property, plant and equipment, net	8,131.6	7,993.5
Advances to affiliates - noncurrent	373.4	229.3
Other noncurrent assets	464.1	466.8
Total other assets	837.5	696.1
Total Assets	9,146.9	8,973.8
Liabilities and Partners' Capital
Payables	87.6	136.4
Payable to affiliates	6.9	7.0
Advances from affiliates	1.4	72.9
Other current liabilities	174.6	175.3
Total current liabilities	270.5	391.6
Long-term debt and capital lease obligation	1,124.7	1,293.6
Payable to affiliate - noncurrent	0.0	0.0
Advances from affiliates - noncurrent	886.4	435.0
Other noncurrent liabilities	199.5	200.0
Total other liabilities and deferred credits	1,085.9	635.0
Total partners' capital	6,665.8	6,653.6
Total Liabilities and Partners' Capital	9,146.9	8,973.8
Eliminations
Assets
Cash and cash equivalents	0.0	0.0	$ 0.0	$ 0.0
Receivables	0.0	0.0
Receivables - affiliate	(6.9)	(7.0)
Gas and liquids stored underground	0.0	0.0
Prepayments	0.0	0.0
Advances to affiliates	(23.8)	(175.6)
Other current assets	(1.2)	(3.1)
Total current assets	(31.9)	(185.7)
Investment in consolidated subsidiaries	(9,268.6)	(9,076.8)
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	(3,342.2)	(2,789.3)
Other noncurrent assets	(0.2)	0.0
Total other assets	(3,342.4)	(2,789.3)
Total Assets	(12,642.9)	(12,051.8)
Liabilities and Partners' Capital
Payables	0.0	0.0
Payable to affiliates	(6.9)	(7.0)
Advances from affiliates	(23.8)	(175.6)
Other current liabilities	(1.4)	(3.1)
Total current liabilities	(32.1)	(185.7)
Long-term debt and capital lease obligation	0.0	0.0
Payable to affiliate - noncurrent	0.0	0.0
Advances from affiliates - noncurrent	(3,342.2)	(2,789.3)
Other noncurrent liabilities	0.0	0.0
Total other liabilities and deferred credits	(3,342.2)	(2,789.3)
Total partners' capital	(9,268.6)	(9,076.8)
Total Liabilities and Partners' Capital	$ (12,642.9)	$ (12,051.8)