Guarantee of Securities of Subsidiaries Balance Sheets (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015	Dec. 31, 2014
Condensed Financial Statements, Captions [Line Items]
Subsidiary or Equity Method Investee, 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 have no significant restrictions on their ability to pay distributions or make loans to the Partnership except as noted in the debt covenants and have no restricted assets at September 30, 2016, and December 31, 2015.
Assets
Cash and cash equivalents	$ 111.4	3.1	$ 5.4	$ 6.6
Receivables	119.5	129.5
Receivables - affiliate	0.0	0.0
Gas and liquids stored underground	12.4	10.7
Prepayments	20.1	16.9
Advances to affiliates	0.0	0.0
Other current assets	8.9	9.6
Total current assets	272.3	169.8
Investment in consolidated subsidiaries	0.0	0.0
Property, plant and equipment, gross	10,156.2	9,706.6
Less—accumulated depreciation and amortization	2,258.0	2,052.2
Property, plant and equipment, net	7,898.2	7,654.4
Other noncurrent assets	472.2	476.1
Advances to affiliates - noncurrent	0.0	0.0
Total other assets	472.2	476.1
Total Assets	8,642.7	8,300.3
Liabilities and Partners' Capital
Payables	118.2	118.6
Payable to affiliates	1.4	1.3
Advances from affiliates	0.0	0.0
Other current liabilities	211.7	194.5
Total current liabilities	331.3	314.4
Long-term debt and capital lease obligation	3,626.8	3,459.3
Payable to affiliate - noncurrent	16.0	16.0
Advances from affiliates - noncurrent	0.0	0.0
Other noncurrent liabilities	200.5	183.9
Total other liabilities and deferred credits	216.5	199.9
Total partners' capital	4,468.1	4,326.7
Total Liabilities and Partners' Capital	8,642.7	8,300.3
Reportable Legal Entities | Parent Guarantor
Assets
Cash and cash equivalents	0.3	0.0	0.0	0.5
Receivables	0.0	0.0
Receivables - affiliate	0.0	0.0
Gas and liquids stored underground	0.0	0.0
Prepayments	0.4	0.2
Advances to affiliates	0.0	0.0
Other current assets	0.0	0.0
Total current assets	0.7	0.2
Investment in consolidated subsidiaries	2,344.6	2,153.5
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
Other noncurrent assets	0.0	0.4
Advances to affiliates - noncurrent	2,140.5	2,190.2
Total other assets	2,140.5	2,190.6
Total Assets	4,485.8	4,344.3
Liabilities and Partners' Capital
Payables	0.3	0.3
Payable to affiliates	1.4	1.3
Advances from affiliates	0.0	0.0
Other current liabilities	0.0	0.0
Total current liabilities	1.7	1.6
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	4,468.1	4,326.7
Total Liabilities and Partners' Capital	4,485.8	4,344.3
Reportable Legal Entities | Subsidiary Issuer
Assets
Cash and cash equivalents	107.3	0.3	1.6	1.8
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	0.0	21.0
Other current assets	0.0	0.0
Total current assets	107.3	21.3
Investment in consolidated subsidiaries	6,608.7	7,067.6
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
Other noncurrent assets	3.4	3.0
Advances to affiliates - noncurrent	504.1	466.3
Total other assets	507.5	469.3
Total Assets	7,223.5	7,558.2
Liabilities and Partners' Capital
Payables	0.1	0.1
Payable to affiliates	0.0	0.0
Advances from affiliates	127.7	107.7
Other current liabilities	35.0	20.9
Total current liabilities	162.8	128.7
Long-term debt and capital lease obligation	2,513.6	1,972.4
Payable to affiliate - noncurrent	0.0	0.0
Advances from affiliates - noncurrent	2,202.4	3,303.6
Other noncurrent liabilities	0.1	0.0
Total other liabilities and deferred credits	2,202.5	3,303.6
Total partners' capital	2,344.6	2,153.5
Total Liabilities and Partners' Capital	7,223.5	7,558.2
Reportable Legal Entities | Non-Guarantor Subsidiaries
Assets
Cash and cash equivalents	3.8	2.8	3.8	4.3
Receivables	119.5	129.5
Receivables - affiliate	7.0	7.0
Gas and liquids stored underground	12.4	10.7
Prepayments	19.7	16.7
Advances to affiliates	127.7	107.7
Other current assets	20.7	12.8
Total current assets	310.8	287.2
Investment in consolidated subsidiaries	0.0	0.0
Property, plant and equipment, gross	10,155.6	9,706.0
Less—accumulated depreciation and amortization	2,257.4	2,051.6
Property, plant and equipment, net	7,898.2	7,654.4
Other noncurrent assets	471.5	472.7
Advances to affiliates - noncurrent	61.9	1,113.4
Total other assets	533.4	1,586.1
Total Assets	8,742.4	9,527.7
Liabilities and Partners' Capital
Payables	117.8	118.2
Payable to affiliates	7.0	7.0
Advances from affiliates	0.0	21.0
Other current liabilities	191.2	176.8
Total current liabilities	316.0	323.0
Long-term debt and capital lease obligation	1,113.2	1,486.9
Payable to affiliate - noncurrent	0.0	0.0
Advances from affiliates - noncurrent	504.1	466.3
Other noncurrent liabilities	200.4	183.9
Total other liabilities and deferred credits	704.5	650.2
Total partners' capital	6,608.7	7,067.6
Total Liabilities and Partners' Capital	8,742.4	9,527.7
Eliminations
Assets
Cash and cash equivalents	0.0	0.0	$ 0.0	$ 0.0
Receivables	0.0	0.0
Receivables - affiliate	(7.0)	(7.0)
Gas and liquids stored underground	0.0	0.0
Prepayments	0.0	0.0
Advances to affiliates	(127.7)	(128.7)
Other current assets	(11.8)	(3.2)
Total current assets	(146.5)	(138.9)
Investment in consolidated subsidiaries	(8,953.3)	(9,221.1)
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
Other noncurrent assets	(2.7)	0.0
Advances to affiliates - noncurrent	(2,706.5)	(3,769.9)
Total other assets	(2,709.2)	(3,769.9)
Total Assets	(11,809.0)	(13,129.9)
Liabilities and Partners' Capital
Payables	0.0	0.0
Payable to affiliates	(7.0)	(7.0)
Advances from affiliates	(127.7)	(128.7)
Other current liabilities	(14.5)	(3.2)
Total current liabilities	(149.2)	(138.9)
Long-term debt and capital lease obligation	0.0	0.0
Payable to affiliate - noncurrent	0.0	0.0
Advances from affiliates - noncurrent	(2,706.5)	(3,769.9)
Other noncurrent liabilities	0.0	0.0
Total other liabilities and deferred credits	(2,706.5)	(3,769.9)
Total partners' capital	(8,953.3)	(9,221.1)
Total Liabilities and Partners' Capital	$ (11,809.0)	$ (13,129.9)