Derivatives (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2013 MMcf	Dec. 31, 2012
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Gas stored underground		$ 0.9	$ 7.0
Capacity of gas available for sale (in mmcf)		9,700
Book value of additional gas available for sale		22.0
Amount of stored gas hedged with derivatives (in mmcf)		11,700
Notional amount	100.0
Gain (Loss) on Interest Rate Derivative Instruments Not Designated as Hedging Instruments	0.5
Commodity Contract [Member] | Designated as Hedging Instrument [Member] | Other current assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Net		0	0.1
Commodity Contract [Member] | Designated as Hedging Instrument [Member] | Other current liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Net		4.9	0.1
Term loan HP Storage [Member]
Derivatives, Fair Value [Line Items]
Long-term Debt, Gross	$ 200.0