Financial Derivative Instruments - Realized and Unrealized Gains and Losses (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Derivative instrument
Losses on fuel derivatives recorded as Nonoperating Expense	$ (184)	$ (3,220)	$ (6,636)	$ (2,650)
Reclassification from de-designation	1,600		4,400
Derivatives not designated as hedges | Fuel derivative contracts
Derivative instrument
Losses realized at settlement	(2,751)	(3,051)	(5,837)	(5,895)
Reversal of prior period unrealized amounts	5,854	4,767	2,488	8,181
Unrealized losses that will settle in future periods	(3,287)	(4,936)	(3,287)	(4,936)
Losses on fuel derivatives recorded as Nonoperating Expense	$ (184)	$ (3,220)	(6,636)	$ (2,650)
Derivatives designated as hedges | Foreign currency derivatives
Derivative instrument
Expected reclassification of net gain			$ 1,300
Reclassification estimated time period			12 months