Derivative Instruments Derivative instruments (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Derivative [Line Items]
Change In fair value of derivative instruments not designated as hedging instruments recorded as increase (decrease) to regulatory assets	$ (437,000)	$ (8,900,000)
Notional amount of interest rate cash flow hedge derivatives	50,000,000
Maximum length of time hedged in cash flow hedge	12 months
Cash flow hedge gain (loss) to be reclassified within twelve months	10,400,000
Derivative net liability position aggregate fair value for cash flow hedging instruments	6,300,000
Assets needed for immediate settlement aggregate fair value for cash flow heding instruments	$ 6,300,000
Oil swap and or collar [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount	2,600,000
Natural gas swap or and collar [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount	11,000,000