Fair Value Measurements and Derivative Instruments - Balance Sheet (Details) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Asset Derivatives
Asset Derivatives		$ 296,646	$ 320,385
Liability Derivatives
Liability Derivatives		103,206	115,961
Designated as Hedging Instrument
Asset Derivatives
Asset Derivatives	[1]	285,726	298,963
Liability Derivatives
Liability Derivatives	[1]	94,778	105,033
Notional Disclosures
Carrying Amount of the Hedged Assets/(Liabilities)		730,462	749,155
Cumulative amount of Fair Value Hedging Adjustment Included in the Carrying Amount of the Hedged Assets/(Liabilities)		(36,648)	(34,813)
Not Designated as Hedging Instrument
Asset Derivatives
Asset Derivatives		10,920	21,422
Liability Derivatives
Liability Derivatives		8,428	10,928
Interest rate swaps | Designated as Hedging Instrument | Other assets
Asset Derivatives
Asset Derivatives	[1]	49,418	7,330
Interest rate swaps | Designated as Hedging Instrument | Other long-term liabilities
Liability Derivatives
Liability Derivatives	[1]	51,917	46,509
Foreign currency forward contracts | Designated as Hedging Instrument | Other assets
Asset Derivatives
Asset Derivatives	[1]	72,830	158,879
Foreign currency forward contracts | Designated as Hedging Instrument | Derivative financial instruments
Asset Derivatives
Asset Derivatives	[1]	21,921	68,352
Liability Derivatives
Liability Derivatives	[1]	26,677	0
Foreign currency forward contracts | Designated as Hedging Instrument | Other long-term liabilities
Liability Derivatives
Liability Derivatives	[1]	4,502	6,625
Foreign currency forward contracts | Not Designated as Hedging Instrument | Other assets
Asset Derivatives
Asset Derivatives		734	2,793
Foreign currency forward contracts | Not Designated as Hedging Instrument | Derivative financial instruments
Asset Derivatives
Asset Derivatives		1,930	9,945
Liability Derivatives
Liability Derivatives		0	2,933
Foreign currency forward contracts | Not Designated as Hedging Instrument | Other long-term liabilities
Liability Derivatives
Liability Derivatives		0	1,139
Fuel swaps | Designated as Hedging Instrument | Other assets
Asset Derivatives
Asset Derivatives	[1]	79,191	51,265
Fuel swaps | Designated as Hedging Instrument | Derivative financial instruments
Asset Derivatives
Asset Derivatives	[1]	62,366	13,137
Liability Derivatives
Liability Derivatives	[1]	9,098	38,488
Fuel swaps | Designated as Hedging Instrument | Other long-term liabilities
Liability Derivatives
Liability Derivatives	[1]	2,584	13,411
Fuel swaps | Not Designated as Hedging Instrument | Other assets
Asset Derivatives
Asset Derivatives		0	798
Fuel swaps | Not Designated as Hedging Instrument | Derivative financial instruments
Asset Derivatives
Asset Derivatives		8,256	7,886
Liability Derivatives
Liability Derivatives		8,428	6,043
Fuel swaps | Not Designated as Hedging Instrument | Other long-term liabilities
Liability Derivatives
Liability Derivatives		$ 0	$ 813

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