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, 2018
Dec. 31, 2017
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value $ 12,125 $ 176,455
Derivative liabilities, fair value 116,352 6,516
Designated as Hedging Instrument    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 17,217 177,031
Derivative liabilities, fair value 117,879 13,121
Designated as Hedging Instrument | Fuel contracts | Prepaid expenses and other assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 2,583 19,220
Derivative liabilities, fair value 1 2,406
Designated as Hedging Instrument | Fuel contracts | Other long-term assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 197 19,854
Derivative liabilities, fair value 29 3,469
Designated as Hedging Instrument | Fuel contracts | Accrued expenses and other liabilities    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 1,173 0
Derivative liabilities, fair value 19,547 3,348
Designated as Hedging Instrument | Fuel contracts | Other long-term liabilities    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 933 576
Derivative liabilities, fair value 51,184 2,148
Designated as Hedging Instrument | Interest rate contracts | Prepaid expenses and other assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 519 0
Derivative liabilities, fair value 0 0
Designated as Hedging Instrument | Interest rate contracts | Other long-term assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 27 0
Derivative liabilities, fair value 0 0
Designated as Hedging Instrument | Interest rate contracts | Accrued expenses and other liabilities    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 0 0
Derivative liabilities, fair value 0 1,020
Designated as Hedging Instrument | Foreign currency contracts | Prepaid expenses and other assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 5,285 52,300
Derivative liabilities, fair value 1,497 730
Designated as Hedging Instrument | Foreign currency contracts | Other long-term assets    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 3,514 85,081
Derivative liabilities, fair value 0 0
Designated as Hedging Instrument | Foreign currency contracts | Accrued expenses and other liabilities    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 112 0
Derivative liabilities, fair value 5,145 0
Designated as Hedging Instrument | Foreign currency contracts | Other long-term liabilities    
Derivatives, Fair Value [Line Items]    
Derivative assets, fair value 2,874 0
Derivative liabilities, fair value $ 40,476 $ 0