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Fair Value Accounting - Net Changes in Net Fair Value of FTR Assets and Liabilities Classified as Level 3 (Details) - Price Risk Derivative - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]        
Beginning balance $ 25,133 $ 17,943 $ 7,044 $ 7,683
Unrealized gains (losses) 4,520 1,591 6,153 (791)
Purchases 1,251 419 26,734 23,537
Settlements (8,644) (7,066) (17,671) (17,542)
Ending balance $ 22,260 $ 12,887 $ 22,260 $ 12,887