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Fair Value, Level 3 Derivative Instruments (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Derivative assets      
Balance at beginning of period $ 324 $ 276 $ 2,816
Settlements (83,992) (98,791) (79,360)
Realized gains recorded in earnings 83,668 98,515 76,544
Unrealized gains recorded in earnings 1,066 324 276
Balance at end of period $ 1,066 $ 324 $ 276