Derivative Instruments (Summary of Commodity Derivative Activity and Presentation in Condensed Statements of Operations) (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Gains reclassified from accumulated other comprehensive income into natural gas and oil revenues	$ (700)	$ 4,800	$ (1,300)	$ 10,100
(Loss) gain subsequent to hedge accounting recognized in gain on mark-to-market derivatives	$ (7,800)	$ (5,500)	$ (1,100)	$ 400