Supplemental Oil and Gas Disclosures - Unaudited (Schedule of Proved Developed and Undeveloped Oil and Gas Reserve Quantities) (Parenthetical) (Details) - MBoe
MBoe in Thousands
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Reserve Quantities [Line Items]      
Production, Barrels of Oil Equivalents Natural gas volumes have been converted to equivalent oil, condensate and NGLs volumes using a conversion factor of one barrel of oil, condensate or NGLs to six cubic feet of natural gas.    
Percentage of extensions and discoveries from successful drilling results in Marcellus Shale   69.00% 74.00%
Proved Developed and Undeveloped Reserves, Net, Period Increase (Decrease) [2] 53,268 [1] (8,574) (1,729)
Appalchian Basin      
Reserve Quantities [Line Items]      
Proved Developed and Undeveloped Reserves, Net, Period Increase (Decrease) (36,800)    
[1] The 2015 revisions of previous estimates resulted primarily from a 36.8 MMBoe decrease in Appalachian Basin reserves due to the suspension of the Marcellus and Utica Shale drilling programs in 2015 and the significant decrease in the 12-month unweighted arithmetic average of the first-day-of-the-month prices for oil and natural gas as of December 31, 2015 and 2014.
[2] Thousand barrels of oil, condensate or NGLs and natural gas equivalent. Natural gas volumes have been converted to equivalent oil, condensate and NGLs volumes using a conversion factor of one barrel of oil, condensate or NGLs to six cubic feet of natural gas.