Long-Term Debt and Other Borrowings - Schedule of Long-term Debt (Parenthetical) (Detail) $ in Millions			1 Months Ended			3 Months Ended		9 Months Ended	12 Months Ended
	Jun. 28, 2018 USD ($)	Jun. 27, 2018 USD ($) Aircraft	Jun. 30, 2018 USD ($) Aircraft	Feb. 28, 2018 Engine	Dec. 31, 2017 USD ($) Aircraft	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($) Aircraft	Jun. 30, 2018 USD ($) Aircraft	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($) Aircraft	Sep. 30, 2015 USD ($) Aircraft	Sep. 30, 2007 USD ($) Aircraft	Sep. 30, 2005 USD ($) Aircraft	Sep. 30, 2004 USD ($) Aircraft	Sep. 30, 2014 USD ($) Aircraft
Flight Equipment [Member]
Debt Instrument [Line Items]
Debt instrument, face amount											$ 8.3
Long term debt interest rate percentage											5.163%
Flight Equipment Maintenance [Member]
Debt Instrument [Line Items]
Debt instrument, face amount						$ 11.9	$ 11.9		$ 11.9	$ 11.9
Long term debt interest rate description								Three-month LIBOR plus a spread ranging from 2.93% to 2.96%
Flight Equipment Maintenance [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Long term debt interest rate percentage			5.267%					5.267%
Flight Equipment Maintenance [Member] \| Minimum [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate							2.93%
Flight Equipment Maintenance [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Long term debt interest rate percentage			5.297%					5.297%
Flight Equipment Maintenance [Member] \| Maximum [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate						2.96%
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2019 [Member]
Debt Instrument [Line Items]
Debt instrument maturity year								2019	2019
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument maturity year								2022	2022
Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2024 [Member]
Debt Instrument [Line Items]
Debt instrument maturity year								2024	2024
Senior and Subordinated Notes Payable to Secured Parties, Collateralized by the Underlying Aircraft, Due 2027 [Member]
Debt Instrument [Line Items]
Debt instrument maturity year								2027	2027
Notes Payable to Secured Parties, Collateralized by the Underlying Aircraft, Due 2028 [Member]
Debt Instrument [Line Items]
Debt instrument maturity year								2028	2028
Senior and Subordinated Notes Payable to Secured Parties, Collateralized by the Underlying Aircraft, Due 2028 [Member]
Debt Instrument [Line Items]
Debt instrument maturity year								2028	2028
Senior and Subordinated Notes Payable to Secured Parties, Collateralized by the Underlying Aircraft, Due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument maturity year								2022
Notes Payable to Financial Institution, Collateralized by the Underlying Equipment, Due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument maturity year								2022	2022
Notes Payable to Financial Institution, Collateralized by the Underlying Equipment, Due 2022 [Member] \| Flight Equipment [Member]
Debt Instrument [Line Items]
Debt instrument, face amount						$ 99.1			$ 99.1
Long term debt interest rate description								monthly LIBOR Plus 7.25%
Long term debt interest rate percentage			9.342%					9.342%
Notes Payable to Financial Institution, Collateralized by the Underlying Equipment, Due 2022 [Member] \| Flight Equipment [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate									7.25%
Notes Payable to Financial Institution, Collateralized by the Underlying Equipment, Due 2020 [Member] \| Flight Equipment [Member]
Debt Instrument [Line Items]
Debt instrument maturity year								2020	2020
Notes Payable to Financial Institution, Collateralized by the Underlying Equipment, Due 2020 [Member] \| Flight Equipment Maintenance [Member]
Debt Instrument [Line Items]
Debt instrument maturity year								2020	2020
Notes Payable to Financial Institution Due 2020 [Member]
Debt Instrument [Line Items]
Debt instrument maturity year								2020	2020
Notes Payable to Financial Institution Due 2020 [Member] \| Flight Equipment Maintenance [Member]
Debt Instrument [Line Items]
Debt instrument, face amount							$ 10.2			$ 10.2	$ 10.2
Long term debt interest rate description								monthly LIBOR plus 3.07%
Long term debt interest rate percentage			5.162%					5.162%
Notes Payable to Financial Institution Due 2020 [Member] \| Flight Equipment Maintenance [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate										3.07%	3.07%
Notes Payable to Financial Institution Due 2019 [Member]
Debt Instrument [Line Items]
Debt instrument maturity year								2019	2019
Working Capital Draw Loan, Collateralized by Certain Flight Equipment and Spare Parts [Member]
Debt Instrument [Line Items]
Debt instrument, face amount							$ 35.0			$ 35.0
Long term debt interest rate description								One-month LIBOR plus 4.25%
Long term debt interest rate percentage			6.342%					6.342%
Working Capital Draw Loan, Collateralized by Certain Flight Equipment and Spare Parts [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate										4.25%
Other Obligations Due to Financial Institution, Collateralized by the Underlying Equipment, Due 2023 [Member]
Debt Instrument [Line Items]
Debt instrument maturity year								2023
Number of spare engines leased \| Engine				2
Imputed interest				9.13%
Notes Payable to Financial Institution Collateralized By Underlying Equipment Due in 2020 with LIBOR Plus 3.30% [Member] \| Flight Equipment Maintenance [Member]
Debt Instrument [Line Items]
Debt instrument, face amount						$ 25.0			$ 25.0
Long term debt interest rate description								Three-month LIBOR plus 3.30%
Long term debt interest rate percentage			5.637%					5.637%
Notes Payable to Financial Institution Collateralized By Underlying Equipment Due in 2020 with LIBOR Plus 3.30% [Member] \| Flight Equipment Maintenance [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate									3.30%
CRJ-900 [Member]
Debt Instrument [Line Items]
Number of aircraft refinanced \| Aircraft			6		9			6
Number of aircraft financed \| Aircraft		6	9					9
Debt instrument, face amount	$ 69.6	$ 27.5	$ 27.5		$ 74.9			$ 27.5
Long term debt interest rate description	LIBOR plus a spread ranging from 3.50% for the senior promissory notes to 7.50% for the subordinated promissory notes							LIBOR plus 3.50%.
Long term debt basis spread on variable rate		3.50%
CRJ-900 [Member] \| Capital Lease Obligations [Member]
Debt Instrument [Line Items]
Debt instrument, face amount			$ 69.6					$ 69.6
CRJ-900 [Member] \| Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2019 [Member]
Debt Instrument [Line Items]
Number of aircraft financed \| Aircraft													5	9
Debt instrument, face amount													$ 118.0
Long term debt interest rate description								London InterBank Offered Rate ("LIBOR"), plus 3%
Long term debt interest rate percentage			5.092%					5.092%
CRJ-900 [Member] \| Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2019 [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate													3.00%
CRJ-900 [Member] \| Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2022 [Member]
Debt Instrument [Line Items]
Number of aircraft financed \| Aircraft												3
CRJ-900 [Member] \| Senior and Subordinated Notes Payable to Secured Parties, Collateralized by the Underlying Aircraft, Due 2027 [Member]
Debt Instrument [Line Items]
Number of aircraft financed \| Aircraft											7
Debt instrument, face amount											$ 170.2
CRJ-900 [Member] \| Senior and Subordinated Notes Payable to Secured Parties, Collateralized by the Underlying Aircraft, Due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument maturity year								2022
CRJ-900 [Member] \| Senior Notes Due Two Thousand Twenty Seven [Member]
Debt Instrument [Line Items]
Debt instrument, face amount											$ 151.0
Long term debt interest rate description								monthly LIBOR plus 2.71%
Long term debt interest rate percentage			4.802%					4.802%
CRJ-900 [Member] \| Senior Notes Due Two Thousand Twenty Seven [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate											2.71%
CRJ-900 [Member] \| Subordinated Notes Due Two Thousand Twenty Seven [Member]
Debt Instrument [Line Items]
Imputed interest											6.25%
Debt discount											$ 8.1
CRJ-900 [Member] \| Senior Notes due Two Thousand Twenty Two [Member]
Debt Instrument [Line Items]
Debt instrument, face amount					$ 46.9
Long term debt interest rate description					3.50
Long term debt interest rate percentage			5.592%					5.592%
CRJ-900 [Member] \| Subordinated Notes DueTwoThousandTwentyTwo [Member]
Debt Instrument [Line Items]
Long term debt interest rate description								Monthly LIBOR plus 4.50%
Long term debt interest rate percentage			6.592%					6.592%
CRJ-900 [Member] \| Subordinated Notes DueTwoThousandTwentyTwo [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate					4.50%
CRJ-900 [Member] \| Senior Notes Due 2020 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount			$ 67.3					$ 67.3
Long term debt basis spread on variable rate	3.50%		3.50%
Long term debt interest rate percentage			5.837%					5.837%
CRJ-900 [Member] \| Senior Notes Due 2020 [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt interest rate description								Three-month LIBOR plus 3.50%
CRJ-900 [Member] \| Subordinated Notes Due 2020 [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate	7.50%		7.50%
Long term debt interest rate percentage			9.837%					9.837%
CRJ-900 [Member] \| Subordinated Notes Due 2020 [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt interest rate description								Three month LIBOR plus 7.50%
CRJ-700 [Member] \| Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2019 [Member]
Debt Instrument [Line Items]
Number of aircraft financed \| Aircraft														5
CRJ-700 [Member] \| Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2022 [Member]
Debt Instrument [Line Items]
Number of aircraft financed \| Aircraft												3
Five C- R- J-700 and Nine C- R- J-900 [Member] \| Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2019 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount														$ 254.7
Long term debt interest rate description								Monthly LIBOR plus 3%
Long term debt interest rate percentage			5.092%					5.092%
Five C- R- J-700 and Nine C- R- J-900 [Member] \| Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2019 [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate														3.00%
Three C- R- J-900 and Three C- R- J-700 [Member] \| Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2022 [Member]
Debt Instrument [Line Items]
Debt instrument, face amount												$ 120.3
Long term debt interest rate description								Monthly LIBOR plus 2.25%
Long term debt interest rate percentage			4.342%					4.342%
Three C- R- J-900 and Three C- R- J-700 [Member] \| Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2022 [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate												2.25%
Ten C- R- J-900 Aircraft [Member] \| Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2022 [Member]
Debt Instrument [Line Items]
Number of aircraft financed \| Aircraft															10
Debt instrument, face amount															$ 88.4
Long term debt interest rate description								monthly LIBOR plus a spread ranging from 1.95% to 7.25%
Ten C- R- J-900 Aircraft [Member] \| Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2022 [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Long term debt interest rate percentage			4.042%					4.042%
Ten C- R- J-900 Aircraft [Member] \| Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2022 [Member] \| Minimum [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate								1.95%
Ten C- R- J-900 Aircraft [Member] \| Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2022 [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Long term debt interest rate percentage			9.342%					9.342%
Ten C- R- J-900 Aircraft [Member] \| Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2022 [Member] \| Maximum [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate								7.25%
Eight C- R- J-900 Aircraft [Member] \| Notes Payable to Financial Institution, Collateralized by the Underlying Aircraft, Due 2024 [Member]
Debt Instrument [Line Items]
Number of aircraft financed \| Aircraft															8
Debt instrument, face amount															$ 114.5
Long term debt interest rate percentage															5.00%
Ten E-175 Aircraft [Member] \| Notes Payable to Secured Parties, Collateralized by the Underlying Aircraft, Due 2028 [Member]
Debt Instrument [Line Items]
Number of aircraft financed \| Aircraft							10			10
Debt instrument, face amount							$ 246.0			$ 246.0
Ten E-175 Aircraft [Member] \| Notes Payable to Secured Parties, Collateralized by the Underlying Aircraft, Due 2028 [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Long term debt interest rate percentage			4.75%					4.75%
Ten E-175 Aircraft [Member] \| Notes Payable to Secured Parties, Collateralized by the Underlying Aircraft, Due 2028 [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Long term debt interest rate percentage			6.25%					6.25%
Eight E-175 Aircraft [Member] \| Senior and Subordinated Notes Payable to Secured Parties, Collateralized by the Underlying Aircraft, Due 2028 [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Number of aircraft financed \| Aircraft							8			8
Debt instrument, face amount							$ 195.3			$ 195.3
Eight E-175 Aircraft [Member] \| Senior Notes Due 2008 [Member]
Debt Instrument [Line Items]
Long term debt interest rate description								Three-month LIBOR plus a spread ranging from 2.20% to 2.32%
Eight E-175 Aircraft [Member] \| Senior Notes Due 2008 [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Debt instrument, face amount							$ 172.0			$ 172.0
Long term debt interest rate percentage			4.537%				2.32%	4.537%		2.32%
Eight E-175 Aircraft [Member] \| Senior Notes Due 2008 [Member] \| Minimum [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate										2.20%
Eight E-175 Aircraft [Member] \| Senior Notes Due 2008 [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Long term debt interest rate percentage			4.657%					4.657%
Eight E-175 Aircraft [Member] \| Senior Notes Due 2008 [Member] \| Maximum [Member] \| LIBOR [Member]
Debt Instrument [Line Items]
Long term debt basis spread on variable rate										2.32%
Eight E-175 Aircraft [Member] \| Subordinated Notes Due 2008 [Member]
Debt Instrument [Line Items]
Long term debt interest rate percentage			4.50%					4.50%