Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements and Derivatives - Derivatives measured at fair value and disclosed by balance sheet location (Details)

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Fair Value Measurements and Derivatives - Derivatives measured at fair value and disclosed by balance sheet location (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value $ 176,455 $ 20,302
Derivative liabilities, fair value 6,516 238,069
Designated as Hedging Instrument | Fuel Swaps | Other long-term assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 19,854 0
Derivative liabilities, fair value 3,469 0
Designated as Hedging Instrument | Fuel Swaps | Accrued expenses and other liabilities    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 0 0
Derivative liabilities, fair value 3,348 44,271
Designated as Hedging Instrument | Fuel Swaps | Other long-term liabilities    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 576 13,237
Derivative liabilities, fair value 2,148 38,608
Designated as Hedging Instrument | Fuel Swaps | Prepaid expenses and other assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 19,220 20,288
Derivative liabilities, fair value 2,406 0
Designated as Hedging Instrument | Foreign currency forward contracts | Other long-term assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 85,081 14
Derivative liabilities, fair value 0 0
Designated as Hedging Instrument | Foreign currency forward contracts | Accrued expenses and other liabilities    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 0 0
Derivative liabilities, fair value 0 61,788
Designated as Hedging Instrument | Foreign currency forward contracts | Other long-term liabilities    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 0 0
Derivative liabilities, fair value 0 88,920
Designated as Hedging Instrument | Foreign currency forward contracts | Prepaid expenses and other assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 52,300 0
Derivative liabilities, fair value 730 0
Designated as Hedging Instrument | Interest rates swaps | Accrued expenses and other liabilities    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 0 0
Derivative liabilities, fair value 1,020 3,331
Designated as Hedging Instrument | Interest rates swaps | Other long-term liabilities    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 0 0
Derivative liabilities, fair value $ 0 $ 1,151