Nature of Operations and Summary of Significant Accounting Policies - Inventories (Details) $ in Millions	3 Months Ended
Dec. 31, 2016 USD ($) item	Dec. 31, 2017 USD ($)
Inventories
Inventory allowance \| $	$ 40.5	$ 17.1
Number of seats on aircraft \| item	50

The amount of the valuation account as of the balance sheet date which reduces the carrying amount of airline related inventory (including expendable merchandise, goods, commodities, or supplies to be used primarily in air transport of passengers or freight) to net realizable value; takes into consideration such factors as market value, excessive quantities based on expected sales, technological obsolescence, and shrinkage.

No definition available.

Name:	us-gaap_AirlineRelatedInventoryValuationReserves
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant