Risk Management and Hedging Activities (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Derivative [Line Items]
Physical purchase and sale of gas and electricity at fixed prices	$ 0	$ 0
No swaps outstanding, interest rate fair value derivatives	0	0
Interest Rate Swap [Member] | Interest Expense [Member]
Derivative [Line Items]
Interest rate contracts, amount of gain reclassified from AOCI into income	1,188,000
Interest Rate Swap [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Pre-tax gain on cash flow hedges remaining in AOCI	6,900,000
Pre-tax gain on cash flow hedge from AOCI to be reclassified during next 12 months		1,200,000
Commodity Contract [Member] | Forward Contracts [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Natural gas, unrealized gain recognized in regulatory assets	14,884,000	9,400,000
Commodity Contract [Member] | Forward Contracts [Member] | Not Designated as Hedging Instrument [Member] | Accrued Liabilities [Member]
Derivative [Line Items]
Natural gas net derivative liability	$ 5,428,000	$ 20,312,000