Natural Gas Producing Activities (Unaudited) - Reserve Information (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) Bcfe well	Dec. 31, 2014 USD ($) Bcfe	Dec. 31, 2013 USD ($) Bcfe location
Oil and Gas Exploration and Production Industries Disclosures [Abstract]
Unproved property impairments | $	$ 19,700	$ 86,600
Unproved oil and gas property impairments primarily as a result of lease expirations prior to drilling | $	37,400	14,600	$ 14,200
Unproved properties | $	$ 898,270	$ 824,527	$ 450,227
Engineer experience (in years)	18 years
Percentage of total net natural gas, NGL and oil proved reserves reviewed	100.00%
Percentage of proved developed reserves covered in the audit	80.00%
Percentage of audit coverage of remaining properties	20.00%
Wells per case, operated wells | well	200
Wells per case, non-operated wells | well	230
Net (downward) upward revisions (in mcfe)	(2,195)	(301.4)	191.5
Undeveloped reserves removed (in mcfe)	2,168	1,047.2
Period undeveloped locations will not be drilled	5 years	5 years
Proved undeveloped reserve removed due to economic locations (in mcfe)	342
Additional downward revisions associated with previously booked locations with shortened lives due to reduction in commodity prices (in mcfe)	259
Increase in proved developed reserves due to improved performance of producing locations (in mcfe)	386	845.1
Increase in reserves of natural gas equivalents (in mcfe)	2,051	3,276.1	2,046.6
Production of natural gas equivalents (in mcfe)	619	488.1	367.1
Negative revisions in reserves due to removal of undeveloped locations and associated reserves (in mcfe)			349
Number of undeveloped locations and associated reserves that were removed | location			58
Decrease in lateral spacing | location			1