Regulatory Assets and Liabilities (Details 1) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Negative Salvage [Member]
Regulatory Liabilities [Line Items]
RegulatoryLiabilities	$ 203,800,000	$ 158,100,000
Deferred Cash Out [Member]
Regulatory Liabilities [Line Items]
RegulatoryLiabilities	6,200,000	1,200,000
Sentinel Meter Station Depreciation [Member]
Regulatory Liabilities [Line Items]
RegulatoryLiabilities	3,900,000	2,800,000
PostretirementBenefitCostsMember
Regulatory Liabilities [Line Items]
RegulatoryLiabilities	24,700,000	22,000,000
Electric Power Cost Liability [Member]
Regulatory Liabilities [Line Items]
RegulatoryLiabilities	7,700,000	0
OtherLiabilitiesMember
Regulatory Liabilities [Line Items]
RegulatoryLiabilities	1,200,000	700,000
Grossed Up Deferred Taxes On Equity Funds Used During Construction [Member]
Regulatory Assets [Line Items]
Regulatory Assets	83,500,000	85,600,000
AssetRetirementObligationCostsMember
Regulatory Assets [Line Items]
Regulatory Assets	125,100,000	114,400,000
DeferredIncomeTaxChargesMember
Regulatory Assets [Line Items]
Regulatory Assets	9,100,000	10,200,000
Postretirement Benefit Asset [Member]
Regulatory Assets [Line Items]
Regulatory Assets	4,600,000	5,700,000
DeferredFuelCostsMember
Regulatory Assets [Line Items]
Regulatory Assets	29,300,000	26,200,000
Electric Power Cost Asset [Member]
Regulatory Assets [Line Items]
Regulatory Assets	0	3,300,000
OtherAssetsMember
Regulatory Assets [Line Items]
Regulatory Assets	$ 0	$ 400,000