Derivative instruments (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012 MMBTU	Jun. 30, 2013	Jun. 30, 2012 MMBTU	Dec. 31, 2012
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)		$ (209,000)
Derivative instruments with credit-risk 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
Unrealized gains (losses) included in accumulated other comprehensive income (loss) to be transferred to earnings			1,800,000
Amount of time remaining to transfer accumulated other comprehensive income to earnings			18 months
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	1,400,000	7,800,000	1,400,000	7,800,000	6,300,000
Assets needed for immediate settlement, of credit-risk-related contingent features, aggregate fair value	1,400,000	7,800,000	1,400,000	7,800,000	6,300,000
Oil swap and\or collar [Member]
Derivative instruments [Line Items]
Derivative, nonmonetary notional amount	3,100,000	3,700,000	3,100,000	3,700,000	2,600,000
Natural gas swap and\or collar [Member]
Derivative instruments [Line Items]
Derivative, nonmonetary notional amount	21,400,000	9,100,000	21,400,000	9,100,000	11,000,000
Natural gas basis swap [Member]
Derivative instruments [Line Items]
Derivative, nonmonetary notional amount		1,700,000		1,700,000
Interest rate swap [Member]
Derivative instruments [Line Items]
Derivative, notional amount		$ 60,000,000		$ 60,000,000	$ 50,000,000