Derivative instruments (Details) (USD $)	3 Months Ended	9 Months Ended
	Sep. 30, 2012 MMBTU	Sep. 30, 2013	Sep. 30, 2012 MMBTU	Mar. 31, 2013	Dec. 31, 2012
Derivative instruments [Line Items]
Natural gas swap agreement total forward notional volumes in MMBtu	31,000		31,000
Change in fair value of derivative instruments not designated as hedging instruments recorded as increase (decrease) to regulatory assets	$ (175,000)		$ (384,000)
Derivative, net liability position, aggregate fair value	53,000		53,000
Aggregate fair value of assets needed for immediate settlement if credit-risk-related contingent features were triggered	53,000		53,000
Amount of unrealized gain (loss) on hedge at de-designation date to be reclassified into AOCI				1,800,000
Amount of time remaining to transfer accumulated other comprehensive income to earnings		15 months
Cash flow hedge gain (loss) to be reclassified within twelve months from AOCI into earnings		28,000
Derivatives with credit-risk-related contingent feature in net liability position, aggregate fair value	9,900,000	9,900,000	9,900,000		6,300,000
Assets needed for immediate settlement, of credit-risk-related contingent features, aggregate fair value	9,900,000	9,900,000	9,900,000		6,300,000
Oil swap and\or collar [Member]
Derivative instruments [Line Items]
Derivative, nonmonetary notional amount	3,300,000	3,900,000	3,300,000		2,600,000
Natural gas swap and\or collar [Member]
Derivative instruments [Line Items]
Derivative, nonmonetary notional amount	8,200,000	16,500,000	8,200,000		11,000,000
Natural gas basis swap [Member]
Derivative instruments [Line Items]
Derivative, nonmonetary notional amount	874,000		874,000
Interest rate swap [Member]
Derivative instruments [Line Items]
Derivative, notional amount	$ 60,000,000		$ 60,000,000		$ 50,000,000