Fair Value Measurements and Derivative Instruments - Balance Sheet (Details) - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
Asset Derivatives
Asset Derivatives		$ 324,483	$ 320,385
Liability Derivatives
Liability Derivatives		103,349	115,961
Designated as Hedging Instrument
Asset Derivatives
Asset Derivatives	[1]	314,260	298,963
Liability Derivatives
Liability Derivatives	[1]	97,641	105,033
Notional Disclosures
Carrying Amount of the Hedged Liabilities		728,659	749,155
Cumulative amount of Fair Value Hedging Adjustment Included in the Carrying Amount of the Hedged Liabilities		(38,772)	(34,813)
Not Designated as Hedging Instrument
Asset Derivatives
Asset Derivatives		10,223	21,422
Liability Derivatives
Liability Derivatives		5,708	10,928
Interest rate swaps | Designated as Hedging Instrument | Other assets
Asset Derivatives
Asset Derivatives	[1]	58,135	7,330
Interest rate swaps | Designated as Hedging Instrument | Other long-term liabilities
Liability Derivatives
Liability Derivatives	[1]	51,674	46,509
Foreign currency forward contracts | Designated as Hedging Instrument | Other assets
Asset Derivatives
Asset Derivatives	[1]	53,981	158,879
Foreign currency forward contracts | Designated as Hedging Instrument | Derivative financial instruments
Asset Derivatives
Asset Derivatives	[1]	14,063	68,352
Liability Derivatives
Liability Derivatives	[1]	37,269	0
Foreign currency forward contracts | Designated as Hedging Instrument | Other long-term liabilities
Liability Derivatives
Liability Derivatives	[1]	4,161	6,625
Foreign currency forward contracts | Not Designated as Hedging Instrument | Other assets
Asset Derivatives
Asset Derivatives		760	2,793
Foreign currency forward contracts | Not Designated as Hedging Instrument | Derivative financial instruments
Asset Derivatives
Asset Derivatives		1,008	9,945
Liability Derivatives
Liability Derivatives		70	2,933
Foreign currency forward contracts | Not Designated as Hedging Instrument | Other long-term liabilities
Liability Derivatives
Liability Derivatives		23	1,139
Fuel swaps | Designated as Hedging Instrument | Other assets
Asset Derivatives
Asset Derivatives	[1]	104,066	51,265
Fuel swaps | Designated as Hedging Instrument | Derivative financial instruments
Asset Derivatives
Asset Derivatives	[1]	84,015	13,137
Liability Derivatives
Liability Derivatives	[1]	3,848	38,488
Fuel swaps | Designated as Hedging Instrument | Other long-term liabilities
Liability Derivatives
Liability Derivatives	[1]	689	13,411
Fuel swaps | Not Designated as Hedging Instrument | Other assets
Asset Derivatives
Asset Derivatives		608	798
Fuel swaps | Not Designated as Hedging Instrument | Derivative financial instruments
Asset Derivatives
Asset Derivatives		7,847	7,886
Liability Derivatives
Liability Derivatives		5,614	6,043
Fuel swaps | Not Designated as Hedging Instrument | Other long-term liabilities
Liability Derivatives
Liability Derivatives		$ 1	$ 813

[1] Accounting Standard Codification 815-20 “Derivatives and Hedging.”