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Fair Value Measurements - Summary of Changes of Derivative Commodity Instruments in Fair Value (Details) - Derivative Commodity Instruments - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2015
Jun. 30, 2014
Jun. 30, 2015
Jun. 30, 2014
Fair Value, Assets and Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]        
Balance at beginning of period $ 2,413 $ 1,378 $ 24,436 $ 18,289
Realized gains (losses) 661 785 13,814 (2,158)
Unrealized gains (losses) relating to instruments held at the reporting date (16,476) 3,800 (38,499) (13,111)
Settlements during period (661) (756) (13,814) 2,187
Balance at end of period (14,063) 5,207 (14,063) 5,207
Mark-to-market loss included in earnings, unrealized $ 15,800 $ 5,200 $ 20,900 $ 4,200