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Supplemental Gas Data PV-10 Reconciliation (Details)
MMcf in Thousands, $ in Thousands
 12 Months Ended
Dec. 31, 2016
USD ($)
MMcf
Dec. 31, 2015
USD ($)
MMcf
Dec. 31, 2014
USD ($)
MMcf
Dec. 31, 2013
USD ($)
MMcf
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]        
Beginning reserves | MMcf 6,251,648 5,642,989 6,827,616 5,731,214
Revenues $ 11,303,409 $ 11,837,732 $ 28,502,852  
Net changes in sales prices and production costs (172,812) (4,151,684) 517,731  
Net changes in sales prices and production costs (150,819) (589,533) (559,563)  
Net change due to revisions in quantity estimates (35,502) 408,006 151,233  
Net change due to extensions, discoveries and improved recovery (54,628) 157,016 418,775  
Development costs incurred during the period 138,813 666,315 952,733  
Difference in previously estimated development costs compared to actual costs incurred during the period (39,821) 8,911 (102,949)  
Increase Due to Purchases of Minerals in Place 238,819 0 0  
Decrease Due to Sales of Minerals in Place (137,998) 0    
Changes in estimated future development costs (158,000) 374,982 595,221  
Net change in future income taxes 36,513 1,259,744 (798,470)  
Accretion of discount and other 271,248 (98,611) 128,636  
Total standardized measure of discounted net cash flows 955,117 1,019,304 2,984,158 $ 1,680,811
Production costs (5,850,941) (6,584,947) (10,100,868)  
Development costs (1,550,294) (1,220,010) (3,368,621)  
Income tax expense (1,482,826) (1,532,454) (5,711,989)  
Future Net Cash Flows 2,419,348 2,500,321 9,321,374  
Discounted to present value at a 10% annual rate $ (1,464,231) $ (1,481,017) $ (6,337,216)  
Natural Gas [Member]        
Discounted Future Net Cash Flows Relating to Proved Oil and Gas Reserves [Line Items]        
Beginning reserves | MMcf 5,828,398 5,060,214 6,317,600 5,585,107