Derivative Instruments (Details) - Natural gas swaps item in Millions	Jun. 30, 2015 USD ($) item	Dec. 31, 2014 USD ($)
Gas hedges
Derivative liabilities | $	$ 15,942,000	$ 18,914,000
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 | $	$ 16,158,000
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	29.9
2015
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	12.9
2016
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	13.8
2017
Gas hedges
Derivative volume activity that is expected to settle or mature each year (in MMBTUs)	3.2