Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements and Derivatives - Effects of derivatives designated as cash flow hedges (Details 2)

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Fair Value Measurements and Derivatives - Effects of derivatives designated as cash flow hedges (Details 2) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]      
Loss recognized in other income (expense), net - ineffective portion     $ (30,700)
Designated as Hedging Instrument | Cash Flow Hedging | Fuel Swaps      
Derivative Instruments, Gain (Loss) [Line Items]      
Gain (loss) recognized in other comprehensive income (loss) - effective portion $ 50,263 $ 127,470 (173,513)
Loss recognized in other income (expense), net - ineffective portion (317) (12,850) (16,011)
Amount reclassified from accumulated other comprehensive income (loss) into fuel expense $ 29,721 $ 85,448 $ 75,808