Acquisition of HP Storage (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended
	Oct. 29, 2012	Oct. 02, 2012	Sep. 30, 2012	Feb. 01, 2012	Dec. 31, 2011	Sep. 30, 2011	Dec. 31, 2010	Dec. 31, 2011 Previously Reported [Member]	Dec. 31, 2011 HP Storage [Member]	Dec. 31, 2011 Eliminations [Member]		Oct. 15, 2012 Boardwalk Pipelines, LP [Member]	Oct. 02, 2012 Boardwalk Pipelines, LP [Member]	Feb. 01, 2012 Boardwalk Pipelines, LP [Member]	Dec. 31, 2011 Boardwalk Pipelines, LP [Member]	Oct. 02, 2012 Boardwalk Pipelines Holding Company [Member]	Dec. 31, 2011 Boardwalk Pipelines Holding Company [Member]
Recasting Due to Accounting for Entities Under Common Control [Line Items]
Business Acquisition, Date of Acquisition Agreement	Oct. 01, 2012
Debt Instrument, Face Amount		$ 225.0
Business Acquisition, Cost of Acquired Entity, Purchase Price		625
Equity ownership interest acquired (in hundredths)												65.00%	35.00%	80.00%	20.00%	65.00%	80.00%
Cash paid for business acquisition					545.5							269.2	147.6	284.8		269.2
Difference between purchase price and carrying amount of net assets				3.0
Net assets acquired				281.8
ASSETS
Cash and cash equivalents			10.9		21.9	84.2	55.0	11.9	10.0	0
Receivables:
Trade, net			79.0		98.6			98.0	0.6	0
Affiliate			0.5		0			0.3	0	(0.3)	[1]
Other			10.6		22.5			20.2	2.3	0
Gas transportation receivables			6.7		5.8			5.8	0	0
Costs recoverable from customers			4.2		9.8			9.8	0	0
Gas stored underground			6.1		1.7			1.7	0	0
Prepayments			19.5		13.9			13.3	0.6	0
Other current assets			2.1		1.8			1.8	0	0
Total current assets			139.6		176.0			162.8	13.5	(0.3)	[1]
Natural gas transmission and other plant			7,603.7		7,536.0			7,049.7	486.3	0
Construction work in progress			158.3		110.6			110.4	0.2	0
Property, plant and equipment, gross			7,762.0		7,646.6			7,160.1	486.5	0
Less-accumulated depreciation and amortization			1,167.7		999.2			997.1	2.1	0
Property, plant and equipment, net			6,594.3		6,647.4			6,163.0	484.4	0
Goodwill			215.3		215.3			163.5	51.8	0
Gas stored underground			109.8		107.9			107.5	0.4	0
Costs recoverable from customers			15.0		15.3			15.3	0	0
Investment in unconsolidated affiliate					0			70.1	0	(70.1)	[1]
Other			99.8		104.5			88.4	16.1	0
Total other assets			439.9		443.0			444.8	68.3	(70.1)	[1]
Total Assets			7,173.8		7,266.4			6,770.6	566.2	(70.4)	[1]
Payables:
Trade			34.5		44.7			42.8	1.9	0
Affiliates			4.7		3.2			3.2	0.3	(0.3)	[1]
Other			13.7		7.3			6.3	1.0	0
Gas Payables:
Transportation			7.7		5.0			5.0	0	0
Storage			1.1		0.1			0.1	0	0
Accrued taxes, other			63.7		44.2			40.6	3.6	0
Accrued interest			29.4		45.2			45.2	0	0
Accrued payroll and employee benefits			22.5		18.4			18.4	0	0
Deferred income			20.9		9.4			9.4	0	0
Other current liabilities			15.3		25.2			21.0	4.2	0
Total current liabilities			213.5		202.7			192.0	11.0	(0.3)	[1]
Long-term debt			3,063.7		3,298.7			3,098.7	200.0	0
Long-term debt - affiliate			100.0		100.0			100.0	0	0
Total long-term debt			3,163.7		3,398.7			3,198.7	200.0	0
Other Liabilities and Deferred Credits:
Pension liability			20.7		27.3			27.3	0	0
Asset retirement obligation			22.3		19.2			16.7	2.5	0
Provision for other asset retirement			57.7		54.5			54.5	0	0
Payable to affiliate			16.0		16.0			16.0	0	0
Other			54.9		61.0			60.2	0.8	0
Total other liabilities and deferred credits			171.6		178.0			174.7	3.3	0
Commitments and Contingencies
Partners' Capital:
Common units			2,936.7		2,514.1			2,513.8	0	0.3	[1]
Class B units			678.4		678.7			678.7	0	0
General partner			70.7		62.0			62.1	0	(0.1)	[1]
Predecessor equity			0		281.6			0	351.9	(70.3)	[1]
Accumulated other comprehensive loss			(60.8)		(49.4)			(49.4)	0	0
Total partners' capital			3,625.0		3,487.0			3,205.2	351.9	(70.1)	[1]
Total Liabilities and Partners' Capital			$ 7,173.8		$ 7,266.4			$ 6,770.6	$ 566.2	$ (70.4)	[1]

[1] Reflects the elimination of the Partnership’s previously reported 20% ownership interest.