Fair Value Measurements and Derivative Instruments - Balance Sheet (Details) - USD ($) $ in Thousands		Jun. 30, 2019	Dec. 31, 2018
Asset Derivatives
Asset Derivatives		$ 78,441	$ 65,297
Liability Derivatives
Liability Derivatives		185,733	201,812
Designated as Hedging Instrument
Asset Derivatives
Asset Derivatives	[1]	76,738	58,576
Liability Derivatives
Liability Derivatives	[1]	185,698	199,795
Notional Disclosures
Carrying Amount of the Hedged Liabilities		730,374	725,486
Cumulative amount of Fair Value Hedging Adjustment Included in the Carrying Amount of the Hedged Liabilities		(3,019)	(24,766)
Not Designated as Hedging Instrument
Asset Derivatives
Asset Derivatives		1,703	6,721
Liability Derivatives
Liability Derivatives		35	2,017
Interest rate swaps | Designated as Hedging Instrument | Other assets
Asset Derivatives
Asset Derivatives	[1]	0	23,518
Interest rate swaps | Designated as Hedging Instrument | Other long-term liabilities
Liability Derivatives
Liability Derivatives	[1]	64,442	40,467
Foreign currency forward contracts | Designated as Hedging Instrument | Other assets
Asset Derivatives
Asset Derivatives	[1]	9,514	10,844
Foreign currency forward contracts | Designated as Hedging Instrument | Derivative financial instruments
Asset Derivatives
Asset Derivatives	[1]	4,343	4,044
Liability Derivatives
Liability Derivatives	[1]	24,742	39,665
Foreign currency forward contracts | Designated as Hedging Instrument | Other long-term liabilities
Liability Derivatives
Liability Derivatives	[1]	54,632	16,854
Foreign currency forward contracts | Not Designated as Hedging Instrument | Other assets
Asset Derivatives
Asset Derivatives		0	1,579
Foreign currency forward contracts | Not Designated as Hedging Instrument | Derivative financial instruments
Asset Derivatives
Asset Derivatives		0	1,751
Liability Derivatives
Liability Derivatives		0	808
Foreign currency forward contracts | Not Designated as Hedging Instrument | Other long-term liabilities
Liability Derivatives
Liability Derivatives		0	833
Fuel swaps | Designated as Hedging Instrument | Other assets
Asset Derivatives
Asset Derivatives	[1]	30,361	9,204
Fuel swaps | Designated as Hedging Instrument | Derivative financial instruments
Asset Derivatives
Asset Derivatives	[1]	32,520	10,966
Liability Derivatives
Liability Derivatives	[1]	12,956	37,627
Fuel swaps | Designated as Hedging Instrument | Other long-term liabilities
Liability Derivatives
Liability Derivatives	[1]	28,926	65,182
Fuel swaps | Not Designated as Hedging Instrument | Other assets
Asset Derivatives
Asset Derivatives		7	587
Fuel swaps | Not Designated as Hedging Instrument | Derivative financial instruments
Asset Derivatives
Asset Derivatives		1,696	2,804
Liability Derivatives
Liability Derivatives		20	376
Fuel swaps | Not Designated as Hedging Instrument | Other long-term liabilities
Liability Derivatives
Liability Derivatives		$ 15	$ 0

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