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
	Mar. 31, 2016	Mar. 31, 2015
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Gains reclassified from accumulated other comprehensive income into natural gas and oil revenues	$ 200	$ 1,600
Gains subsequent to hedge accounting recognized in gain on mark-to-market derivatives	$ 2,300	$ 2,000