Fair Value Measurements and Derivative Instruments (Balance Sheet) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Asset Derivatives
Asset Derivatives	$ 320,385	$ 19,397
Liability Derivatives
Liability Derivatives	115,961	373,497
Designated as Hedging Instrument
Asset Derivatives
Asset Derivatives	298,963	18,854
Liability Derivatives
Liability Derivatives	105,033	356,029
Not Designated as Hedging Instrument
Asset Derivatives
Asset Derivatives	21,422	543
Liability Derivatives
Liability Derivatives	10,928	17,468
Interest rate swaps | Designated as Hedging Instrument | Other Assets
Asset Derivatives
Asset Derivatives	7,330	5,246
Interest rate swaps | Designated as Hedging Instrument | Other long-term Liabilities
Liability Derivatives
Liability Derivatives	46,509	57,679
Foreign currency forward contracts | Designated as Hedging Instrument | Other Assets
Asset Derivatives
Asset Derivatives	158,879	0
Foreign currency forward contracts | Designated as Hedging Instrument | Derivative Financial Instruments
Asset Derivatives
Asset Derivatives	68,352	0
Liability Derivatives
Liability Derivatives	0	5,574
Foreign currency forward contracts | Designated as Hedging Instrument | Other long-term Liabilities
Liability Derivatives
Liability Derivatives	6,625	68,165
Foreign currency forward contracts | Not Designated as Hedging Instrument | Other Assets
Asset Derivatives
Asset Derivatives	2,793	0
Foreign currency forward contracts | Not Designated as Hedging Instrument | Derivative Financial Instruments
Asset Derivatives
Asset Derivatives	9,945	0
Liability Derivatives
Liability Derivatives	2,933	0
Foreign currency forward contracts | Not Designated as Hedging Instrument | Other long-term Liabilities
Liability Derivatives
Liability Derivatives	1,139	0
Fuel contracts | Designated as Hedging Instrument | Other Assets
Asset Derivatives
Asset Derivatives	51,265	13,608
Fuel contracts | Designated as Hedging Instrument | Derivative Financial Instruments
Asset Derivatives
Asset Derivatives	13,137	0
Liability Derivatives
Liability Derivatives	38,488	129,486
Fuel contracts | Designated as Hedging Instrument | Other long-term Liabilities
Liability Derivatives
Liability Derivatives	13,411	95,125
Fuel contracts | Not Designated as Hedging Instrument | Other Assets
Asset Derivatives
Asset Derivatives	798	543
Fuel contracts | Not Designated as Hedging Instrument | Derivative Financial Instruments
Asset Derivatives
Asset Derivatives	7,886	0
Liability Derivatives
Liability Derivatives	6,043	11,532
Fuel contracts | Not Designated as Hedging Instrument | Other long-term Liabilities
Liability Derivatives
Liability Derivatives	$ 813	$ 5,936