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Fair Value, Level 3 Derivative Instruments (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Derivative assets        
Balance at beginning of period $ 606 $ 1,420 $ 276 $ 2,816
Settlements (17,793) (18,897) (62,313) (57,578)
Realized gains recorded in earnings 17,187 17,477 62,037 54,762
Unrealized gains recorded in earnings 749 420 749 420
Balance at end of period $ 749 $ 420 $ 749 $ 420