Derivative Instruments (Details) (Natural Gas Swaps, USD $)	Mar. 31, 2013 MMBTU	Dec. 31, 2012
Gas hedges
Fair values of price risk derivative assets	$ 576,000	$ 576,000
Fair values of price risk derivative liabilities	$ 1,085,000	$ 1,085,000
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	3,500,000
2013
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	2,200,000
2014
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	1,300,000