Derivative instruments (Details) (Natural gas swaps, USD $)	Sep. 30, 2014 item	Dec. 31, 2013
Gas hedges
Derivative Liabilities	$ (1,764,000)
Derivative assets		1,011,000
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	14,300,000
Letters of credit required to be posted with counterparties, if credit-risk-related contingent features were triggered due to credit rating being downgraded below investment grade	$ 2,022,000
2014
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	400,000
2015
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	7,900,000
2016
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	5,400,000