Derivative instruments (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012 MMBTU	Jun. 30, 2011	Jun. 30, 2012 MMBTU	Jun. 30, 2011
Derivative instruments [Line Items]
Natural gas swap agreement total forward notional volumes in MMBtu	123,000		123,000
Change in fair value of derivative instruments not designated as hedging instruments recorded as increase (decrease) to regulatory assets	$ (261,000)	$ (1,900,000)	$ (209,000)	$ (8,500,000)
Derivative instruments with credit-risk-related contingent features, net liability position, aggregate fair value	228,000		228,000
Aggregate fair value of assets needed for immediate settlement if credit-risk-related contingent features were triggered	228,000		228,000
Maximum length of time hedged in swap, collar and/or put cash flow hedge agreements (in months)			18 months
Notional amount of interest rate derivatives	60,000,000		60,000,000
Amount of gain (loss) of hedge ineffectiveness on oil and natural gas derivatives	3,900,000		(400,000)
Gain on derivatives that did not qualify for hedge accounting	1,000,000	1,900,000	1,000,000	179,000
Cash flow hedge gain (loss) to be reclassified within twelve months from AOCI into earnings			22,100,000
Derivatives with credit-risk-related contingent feature in net liability position, aggregate fair value	7,800,000		7,800,000
Assets needed for immediate settlement, of credit-risk-related contingent features, aggregate fair value	$ 7,800,000		$ 7,800,000
Oil swap and\or collar [Member]
Derivative instruments [Line Items]
Nonmonetary notional amount of derivative instruments	3,700,000		3,700,000
Natural gas swap and\or collar [Member]
Derivative instruments [Line Items]
Nonmonetary notional amount of derivative instruments	9,100,000		9,100,000
Natural gas basis swap [Member]
Derivative instruments [Line Items]
Nonmonetary notional amount of derivative instruments	1,700,000		1,700,000