COMMITMENTS AND CONTINGENCIES - Schedule of Aircraft Orders and Options (Details) - aircraft	3 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017
Commitments
AnalysisOfAircraftOrdersAndOptions [Roll Forward]
Beginning of period	27	29	32
Aircraft delivered	0	(2)	(3)
End of period	27	27	29
Options
AnalysisOfAircraftOrdersAndOptions [Roll Forward]
Beginning of period	4	4	4
Aircraft delivered	0	0	0
End of period	4	4	4