Fair Value Measurements and Derivative Instruments - Derivatives Not Designated as Hedging Instruments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Derivative Instruments
Amount of gain (loss) recognized in income on derivatives	$ 7,810	$ 13,812
Other expense
Derivative Instruments
Amount of gain (loss) recognized in income on derivatives	7,810	13,752
Foreign currency forward contracts | Other expense
Derivative Instruments
Amount of gain (loss) recognized in income on derivatives	5,635	13,812
Fuel swaps | Other expense
Derivative Instruments
Amount of gain (loss) recognized in income on derivatives	(30)	(60)
Fuel swaps | Fuel cost
Derivative Instruments
Amount of gain (loss) recognized in income on derivatives	$ 2,205	$ 0