Quarterly report pursuant to Section 13 or 15(d)

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

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Fair Value Measurements and Derivatives - Derivatives measured at fair value and discloses balance sheet location (Details) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value $ 223,392 $ 6,352
Derivative liabilities, fair value 176,455 6,516
Designated as Hedging Instrument    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 223,582 177,031
Derivative liabilities, fair value 13,664 13,121
Fuel swaps | Designated as Hedging Instrument | Prepaid expenses and other assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 17,637 19,220
Derivative liabilities, fair value 3,089 2,406
Fuel swaps | Designated as Hedging Instrument | Other long-term assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 14,540 19,854
Derivative liabilities, fair value 4,152 3,469
Fuel swaps | Designated as Hedging Instrument | Accrued expenses and other liabilities    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 60 0
Derivative liabilities, fair value 3,107 3,348
Fuel swaps | Designated as Hedging Instrument | Other long-term liabilities    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 130 576
Derivative liabilities, fair value 2,913 2,148
Foreign currency forward contracts | Designated as Hedging Instrument | Prepaid expenses and other assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 78,438 52,300
Derivative liabilities, fair value 71 730
Foreign currency forward contracts | Designated as Hedging Instrument | Other long-term assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 112,777 85,081
Derivative liabilities, fair value 0 0
Interest rate swaps | Designated as Hedging Instrument | Accrued expenses and other liabilities    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 0 0
Derivative liabilities, fair value $ 332 $ 1,020