Derivative instruments (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012 MMBTU	Dec. 31, 2012
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
Notional amount of interest rate derivatives	40,000,000	60,000,000	50,000,000
Cash flow hedge gain (loss) to be reclassified within twelve months from AOCI into earnings	1,500,000
Derivatives with credit-risk-related contingent feature in net liability position, aggregate fair value	12,400,000
Assets needed for immediate settlement, of credit-risk-related contingent features, aggregate fair value	$ 12,400,000
Maximum length of time hedged in swap, collar and/or put cash flow hedge agreements (in months)	33 months
Oil swap and\or collar [Member]
Derivative instruments [Line Items]
Derivative, nonmonetary notional amount	2,800,000	4,000,000	2,600,000
Natural gas swap and\or collar [Member]
Derivative instruments [Line Items]
Derivative, nonmonetary notional amount	25,900,000	10,900,000	11,000,000
Natural gas basis swap [Member]
Derivative instruments [Line Items]
Derivative, nonmonetary notional amount	2,600,000