Derivative instruments (Details) (USD $)	3 Months Ended
	Mar. 31, 2012 MMBTU	Mar. 31, 2011
Derivative instruments [Line Items]
Natural gas swap agreement total forward notional volumes in MMBtu	214,000
Change in fair value of derivative instruments not designated as hedging instruments recorded as increase (decrease) to regulatory assets	$ 52,000	$ (6,600,000)
Derivative instruments with credit-risk-related contingent features, net liability position, aggregate fair value	489,000
Aggregate fair value of assets needed for immediate settlement if credit-risk-related contingent features were triggered	489,000
Maximum length of time hedged in swap, collar and/or put cash flow hedge agreements (in months)	21
Notional amount of interest rate derivatives	60,000,000
Amount of loss due to hedge ineffectiveness	4,300,000
Loss on derivatives that did not qualify for hedge accounting		1,700,000
Cash flow hedge gain (loss) to be reclassified within twelve months from AOCI into earnings	5,300,000
Derivatives with credit-risk-related contingent feature in net liability position, aggregate fair value	29,200,000
Assets needed for immediate settlement, of credit-risk-related contingent features, aggregate fair value	$ 29,200,000
Oil swap and\or collar [Member]
Derivative instruments [Line Items]
Nonmonetary notional amount - derivative instruments	4,000,000
Natural gas swap and\or collar [Member]
Derivative instruments [Line Items]
Nonmonetary notional amount of derivative instruments	10,900,000
Natural gas basis swap [Member]
Derivative instruments [Line Items]
Nonmonetary notional amount of derivative instruments	2,600,000