Derivative Instruments (Summary of Gains or Losses Recognized Within Statements of Operations for Derivative Instruments Previously Designated as Cash Flow Hedges) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Derivative Instruments And Hedging Activities Disclosure [Abstract]
(Loss) gain reclassified from accumulated other comprehensive income into natural gas and oil revenues	$ (1,600)	$ (100)	$ 2,800	$ 6,700
Gain subsequent to December 31, 2014 recognized in gain on mark-to-market derivatives	$ 6,100		$ 6,300