Fair Value Measurements and Derivative Instruments - Derivatives Not Designated as Hedging Instruments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative Instruments
Amount of gain (loss) recognized in income on derivatives	$ (11,647)	$ 21,776	$ (41,397)	$ 56,836
Foreign currency forward contracts | Other income (expense)
Derivative Instruments
Amount of gain (loss) recognized in income on derivatives	(12,097)	21,951	(43,356)	57,019
Fuel swaps | Other income (expense)
Derivative Instruments
Amount of gain (loss) recognized in income on derivatives	(28)	(175)	155	(255)
Fuel swaps | Fuel cost
Derivative Instruments
Amount of gain (loss) recognized in income on derivatives	$ 478	$ 0	$ 1,804	$ 0