SCHEDULE II—VALUATION AND QUALIFYING ACCOUNTS (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2016 USD ($) item	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
VALUATION AND QUALIFYING ACCOUNTS
Balance at Beginning of Year		$ 40,670	$ 14,120	$ 11,914
Additions Charged to Costs and Expenses			26,564	2,345
Deductions		(23,415)	(14)	(139)
Balance at End of Year	$ 40,670	17,255	40,670	14,120
Number of seats on aircraft | item	50
Allowance for inventory obsolescence
VALUATION AND QUALIFYING ACCOUNTS
Balance at Beginning of Year		40,497	13,933	11,588
Additions Charged to Costs and Expenses			26,564	2,345
Deductions		(23,399)
Balance at End of Year	$ 40,497	17,098	40,497	13,933
Allowance for doubtful accounts receivable
VALUATION AND QUALIFYING ACCOUNTS
Balance at Beginning of Year		173	187	326
Deductions		(16)	(14)	(139)
Balance at End of Year	$ 173	$ 157	$ 173	$ 187