Fair Value Measurements and Derivative Instruments - Derivatives Not Designated as Hedging Instruments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Derivative Instruments
Amount of gain (loss) recognized in income on derivatives	$ (37,562)	$ 21,236	$ (29,750)	$ 34,988
Foreign currency forward contracts | Other income (expense)
Derivative Instruments
Amount of gain (loss) recognized in income on derivatives	(36,894)	21,256	(31,259)	35,068
Fuel swaps | Other income (expense)
Derivative Instruments
Amount of gain (loss) recognized in income on derivatives	213	(20)	183	(80)
Fuel swaps | Fuel cost
Derivative Instruments
Amount of gain (loss) recognized in income on derivatives	$ (881)	$ 0	$ 1,326	$ 0