Derivatives (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2011		Jun. 30, 2010		Jun. 30, 2011		Jun. 30, 2010		Dec. 31, 2010
Derivatives [Abstract]
Gas stored underground	$ 10.0				$ 10.0				$ 3.6
Additional gas available for sale (in BcF)	3.1				3.1				4.5
Book value of additional gas available for sale	6.9				6.9				8.8
Volume of natural gas and cash for fuel reimbursement hedged (in Bcf)	3.8				3.8
Derivatives, Fair Value [Line Items]
Net loss to be reclassified into earnings within the next twelve months					1.8
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion, amount of gain/(loss) recognized in AOCI on derivatives	1.1		(2.5)		1.4		6.3
Effective portion, amount of gain/(loss) reclassified from AOCI into income	(0.6)		2.5		(0.9)		3.8
Commodity Contract [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion, amount of gain/(loss) recognized in AOCI on derivatives	1.1		(2.5)		1.4		6.3
Commodity Contract [Member] | Other Assets [Member]
Derivatives, Fair Value [Line Items]
Asset derivatives fair value	0.2				0.2				0
Commodity Contract [Member] | Other Liabilities [Member]
Derivatives, Fair Value [Line Items]
Liability derivatives fair value	0.3				0.3				1.7
Interest Rate Contract [Member] | Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion, amount of gain/(loss) recognized in AOCI on derivatives	0	[1]	0	[1]	0	[1]	0	[1]
Cash Flow Hedging [Member] | Operating Revenue [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion, amount of gain/(loss) reclassified from AOCI into income			2.9	[2]	0.1	[3]	4.6	[4]
Cash Flow Hedging [Member] | Interest Expense [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion, amount of gain/(loss) reclassified from AOCI into income	(0.4)		(0.4)		(0.8)		(0.8)
Cash Flow Hedging [Member] | Net loss on disposal of operating assets [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Effective portion, amount of gain/(loss) reclassified from AOCI into income	$ (0.2)				$ (0.2)

[1] Related to amounts deferred in AOCI from Treasury rate locks used in hedging interest payments associated with debt offerings that were settled in previous periods and are being amortized to earnings over the terms of the related interest payments, generally the terms of the related debt.

[2] $1.3 million was recorded in Gas transportation revenues and $1.6 million was recorded in Other revenues.

[3] Recorded in Gas transportation revenues.

[4] $2.3 million was recorded in Gas transportation revenues and $2.3 million was recorded in Other revenues.