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Financial Instruments - Schedule of Changes in Fair Value of Financial Assets and Liabilities Classified as Level 3 (Detail) - Significant Unobservable Inputs (Level 3) [Member]
$ in Thousands
6 Months Ended
Jun. 30, 2018
USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]  
Net Liabilities at beginning of period $ (3,034)
Total realized and unrealized gains (losses):  
Included in earnings (7,533) [1]
Purchases, sales, issuances and settlements 1,576
Net Liabilities at end of period $ (8,991)
[1] Derivative instruments are reported in revenues as realized gain (loss) and on a separately reported line item captioned unrealized gain (loss) on derivative instruments.