Annual report pursuant to Section 13 and 15(d)

Derivative Instruments and Hedging Activities (Schedule of Gain or Loss on Derivative Contracts) (Details)

v3.10.0.1
Derivative Instruments and Hedging Activities (Schedule of Gain or Loss on Derivative Contracts) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]      
Net gain (loss) on fair value adjustments $ 75,816 $ (10,429) $ (38,135)
Total gain (loss) on oil & natural gas derivatives 48,544 (18,901) (20,233)
Not Designated as Hedging Instrument      
Derivative Instruments, Gain (Loss) [Line Items]      
Total gain (loss) on oil & natural gas derivatives 48,544 (18,901) (20,233)
Not Designated as Hedging Instrument | Oil      
Derivative Instruments, Gain (Loss) [Line Items]      
Net gain (loss) on settlements (27,510) (9,067) 17,801
Net gain (loss) on fair value adjustments 72,973 (11,426) (37,543)
Total gain (loss) on oil & natural gas derivatives 45,463 (20,493) (19,742)
Not Designated as Hedging Instrument | Natural gas      
Derivative Instruments, Gain (Loss) [Line Items]      
Net gain (loss) on settlements 238 594 102
Net gain (loss) on fair value adjustments 2,843 998 (593)
Total gain (loss) on oil & natural gas derivatives $ 3,081 $ 1,592 $ (491)