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
Jun. 30, 2018
Dec. 31, 2017
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value $ 128,211 $ 176,455
Derivative liabilities, fair value 4,760 6,516
Designated as Hedging Instrument    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 128,211 177,031
Derivative liabilities, fair value 8,378 13,121
Fuel contracts | Designated as Hedging Instrument | Prepaid expenses and other assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 48,058 19,220
Derivative liabilities, fair value 0 2,406
Fuel contracts | Designated as Hedging Instrument | Other long-term assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 32,482 19,854
Derivative liabilities, fair value 658 3,469
Fuel contracts | Designated as Hedging Instrument | Accrued expenses and other liabilities    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 0 0
Derivative liabilities, fair value 0 3,348
Fuel contracts | Designated as Hedging Instrument | Other long-term liabilities    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 0 576
Derivative liabilities, fair value 0 2,148
Foreign currency forward contracts | Designated as Hedging Instrument | Prepaid expenses and other assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 3,502 52,300
Derivative liabilities, fair value 0 730
Foreign currency forward contracts | Designated as Hedging Instrument | Other long-term assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 42,186 85,081
Derivative liabilities, fair value 2,960 0
Foreign currency forward contracts | Designated as Hedging Instrument | Other long-term liabilities    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 0 0
Derivative liabilities, fair value 4,760 0
Interest rate contracts | Designated as Hedging Instrument | Prepaid expenses and other assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 621 0
Derivative liabilities, fair value 0 0
Interest rate contracts | Designated as Hedging Instrument | Other long-term assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 1,362 0
Derivative liabilities, fair value 0 0
Interest rate contracts | Designated as Hedging Instrument | Accrued expenses and other liabilities    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 0 0
Derivative liabilities, fair value $ 0 $ 1,020