Guarantee of Securities of Subsidiaries Balance Sheets (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Dec. 31, 2013
Condensed Financial Statements, Captions [Line Items]
Equity Restrictions	$ 0.0
Other Restricted Assets	$ 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 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, 2015.
Assets
Cash and cash equivalents	$ 5.4	$ 6.6	$ 6.2	$ 28.5
Receivables	103.7	110.9
Receivables - affiliate	0.0	0.0
Gas and liquids stored underground	10.1	4.1
Prepayments	19.1	14.5
Advances to affiliates	0.0	0.0
Other current assets	6.9	13.5
Total current assets	145.2	149.6
Investment in consolidated subsidiaries	0.0	0.0
Property, plant and equipment, gross	9,595.4	9,355.6
Less—accumulated depreciation and amortization	1,998.7	1,766.4
Property, plant and equipment, net	7,596.7	7,589.2
Other noncurrent assets	488.0	468.0
Advances to affiliates - noncurrent	0.0	0.0
Total other assets	488.0	468.0
Total Assets	8,229.9	8,206.8
Liabilities and Partners' Capital
Payables	86.5	62.1
Payable to affiliates	1.4	1.5
Advances from affiliates	0.0	0.0
Other current liabilities	187.1	156.6
Total current liabilities	275.0	220.2
Long-term debt and capital lease obligation	3,459.2	3,689.7
Payable to affiliate - noncurrent	16.0	16.0
Advances from affiliates - noncurrent	0.0	0.0
Other noncurrent liabilities	184.1	178.6
Total other liabilities and deferred credits	200.1	194.6
Total partners' capital	4,295.6	4,102.3
Total Liabilities and Partners' Capital	8,229.9	8,206.8
Eliminations
Assets
Cash and cash equivalents	0.0	0.0	0.0	0.0
Receivables	0.0	0.0
Receivables - affiliate	(6.6)	(9.0)
Gas and liquids stored underground	0.0	0.0
Prepayments	0.0	0.0
Advances to affiliates	(198.1)	(112.5)
Other current assets	(4.5)	(6.2)
Total current assets	(209.2)	(127.7)
Investment in consolidated subsidiaries	(9,087.3)	(8,714.7)
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
Other noncurrent assets	0.0	(0.6)
Advances to affiliates - noncurrent	(3,680.0)	(3,356.8)
Total other assets	(3,680.0)	(3,357.4)
Total Assets	(12,976.5)	(12,199.8)
Liabilities and Partners' Capital
Payables	0.0	0.0
Payable to affiliates	(6.6)	(9.0)
Advances from affiliates	(198.1)	(112.5)
Other current liabilities	(4.5)	(6.5)
Total current liabilities	(209.2)	(128.0)
Long-term debt and capital lease obligation	0.0	0.0
Payable to affiliate - noncurrent	0.0	0.0
Advances from affiliates - noncurrent	(3,680.0)	(3,356.8)
Other noncurrent liabilities	0.0	(0.3)
Total other liabilities and deferred credits	(3,680.0)	(3,357.1)
Total partners' capital	(9,087.3)	(8,714.7)
Total Liabilities and Partners' Capital	(12,976.5)	(12,199.8)
Parent Guarantor | Reportable Legal Entities [Member]
Assets
Cash and cash equivalents	0.0	0.5	0.0	0.2
Receivables	0.0	0.0
Receivables - affiliate	0.0	0.0
Gas and liquids stored underground	0.0	0.0
Prepayments	0.1	0.1
Advances to affiliates	0.0	0.0
Other current assets	0.2	0.5
Total current assets	0.3	1.1
Investment in consolidated subsidiaries	2,104.4	1,970.6
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	0.0
Advances to affiliates - noncurrent	2,209.2	2,148.3
Total other assets	2,209.2	2,148.3
Total Assets	4,313.9	4,120.0
Liabilities and Partners' Capital
Payables	0.9	0.2
Payable to affiliates	1.4	1.5
Advances from affiliates	0.0	0.0
Other current liabilities	0.0	0.0
Total current liabilities	2.3	1.7
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	4,295.6	4,102.3
Total Liabilities and Partners' Capital	4,313.9	4,120.0
Subsidiary Issuer | Reportable Legal Entities [Member]
Assets
Cash and cash equivalents	1.6	1.8	0.7	9.2
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	25.0	6.3
Other current assets	0.0	0.0
Total current assets	26.6	8.1
Investment in consolidated subsidiaries	6,982.9	6,744.1
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	5.2	3.4
Advances to affiliates - noncurrent	469.8	212.0
Total other assets	475.0	215.4
Total Assets	7,484.5	6,967.6
Liabilities and Partners' Capital
Payables	0.1	0.1
Payable to affiliates	0.0	0.0
Advances from affiliates	173.1	106.2
Other current liabilities	23.0	21.4
Total current liabilities	196.2	127.7
Long-term debt and capital lease obligation	1,973.7	1,724.5
Payable to affiliate - noncurrent	0.0	0.0
Advances from affiliates - noncurrent	3,210.2	3,144.8
Other noncurrent liabilities	0.0	0.0
Total other liabilities and deferred credits	3,210.2	3,144.8
Total partners' capital	2,104.4	1,970.6
Total Liabilities and Partners' Capital	7,484.5	6,967.6
Non-Guarantor Subsidiaries | Reportable Legal Entities [Member]
Assets
Cash and cash equivalents	3.8	4.3	$ 5.5	$ 19.1
Receivables	103.7	110.9
Receivables - affiliate	6.6	9.0
Gas and liquids stored underground	10.1	4.1
Prepayments	19.0	14.4
Advances to affiliates	173.1	106.2
Other current assets	11.2	19.2
Total current assets	327.5	268.1
Investment in consolidated subsidiaries	0.0	0.0
Property, plant and equipment, gross	9,594.8	9,355.0
Less—accumulated depreciation and amortization	1,998.1	1,765.8
Property, plant and equipment, net	7,596.7	7,589.2
Other noncurrent assets	482.8	465.2
Advances to affiliates - noncurrent	1,001.0	996.5
Total other assets	1,483.8	1,461.7
Total Assets	9,408.0	9,319.0
Liabilities and Partners' Capital
Payables	85.5	61.8
Payable to affiliates	6.6	9.0
Advances from affiliates	25.0	6.3
Other current liabilities	168.6	141.7
Total current liabilities	285.7	218.8
Long-term debt and capital lease obligation	1,485.5	1,965.2
Payable to affiliate - noncurrent	0.0	0.0
Advances from affiliates - noncurrent	469.8	212.0
Other noncurrent liabilities	184.1	178.9
Total other liabilities and deferred credits	653.9	390.9
Total partners' capital	6,982.9	6,744.1
Total Liabilities and Partners' Capital	$ 9,408.0	$ 9,319.0