Fair Value Measurements and Derivative Instruments (Derivatives Not Designated as Hedging Instruments) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Derivative instruments disclosure
Amount of Gain (Loss) Recognized in Income on Derivative		$ (61,148)	$ 61,704	$ (45,670)
Foreign exchange contracts | Other income (expense)
Derivative instruments disclosure
Amount of Gain (Loss) Recognized in Income on Derivative	$ (51,029)	(62,423)	61,952
Fuel contracts | Other income (expense)
Derivative instruments disclosure
Amount of Gain (Loss) Recognized in Income on Derivative	(1,000)	114	(1,133)
Fuel contracts | Fuel
Derivative instruments disclosure
Amount of Gain (Loss) Recognized in Income on Derivative	$ 0	$ 1,161	$ 0