Derivative Instruments (Details) (Natural Gas Swaps, USD $)	Jun. 30, 2013 MMBTU	Dec. 31, 2012
Gas hedges
Fair values of price risk derivative liabilities	$ 3,114,000	$ 1,085,000
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	9,000,000
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	$ 2,761,000
2013
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	7,300,000
2014
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	1,600,000
2015
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	100,000