Derivative instruments: (Details) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Gas hedges
Fair value of price risk derivative liabilities	$ 1,085,000	$ 7,220,000
Natural Gas Swaps
Gas hedges
Fair value of price risk derivative liabilities	1,085,000	7,220,000
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	5,400,000
Natural Gas Swaps | Maximum
Gas hedges
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	$ 909,000
Natural Gas Swaps | 2013
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	4,400,000
Natural Gas Swaps | 2014
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	1,000,000