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Summary of Significant Accounting Policies - Additional Information (Detail)
 3 Months Ended 12 Months Ended
Sep. 30, 2018
USD ($)
Jun. 30, 2018
USD ($)
Mar. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Sep. 30, 2017
USD ($)
Jun. 30, 2017
USD ($)
Mar. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Sep. 30, 2018
USD ($)
Segment
Sep. 30, 2017
USD ($)
Sep. 30, 2016
USD ($)
Sep. 30, 2015
USD ($)
Summary Of Significant Accounting Policies [Line Items]                        
Number of operating segments | Segment                 1      
Number of reportable segments | Segment                 1      
Marketable securities $ 19,921,000       $ 0       $ 19,921,000 $ 0    
Outstanding letters of credit to be collateralized by amounts on deposit, classified as restricted cash 3,823,000       3,559,000       3,823,000 3,559,000    
Allowance for obsolete expendable parts and supplies                 200,000 419,000 $ 41,000  
Reduction of unfavorable lease liability                 6,600,000 6,800,000 6,800,000  
Write off of unfavorable lease liability                 1,200,000      
Total operating revenues $ 177,532,000 $ 171,739,000 $ 167,640,000 $ 164,684,000 $ 157,293,000 $ 166,952,000 $ 159,096,000 $ 160,235,000 681,595,000 643,576,000 587,836,000  
Engine overhaul expense                 51,200,000 64,000,000 90,900,000  
Engine overhaul pass-through expense                 12,300,000 300,000 0  
Airframe check expense                 21,500,000 22,600,000 13,200,000  
Airframe check pass-through expense                 7,500,000 4,900,000 0  
Change in accounting policy , effect of increase in SARs and Phantom Stock liability                 2,400,000      
Payment received from former vendor for engine maintenance support credits                   4,600,000    
Correction of prior period adjustment to increase retained earnings                       $ 2,300,000
Contract Revenue [Member] | Rental Income [Member]                        
Summary Of Significant Accounting Policies [Line Items]                        
Total operating revenues                 217,000,000 217,600,000 $ 190,100,000  
Property and Equipment [Member]                        
Summary Of Significant Accounting Policies [Line Items]                        
Impairment charges                 0 0    
Aircraft [Member]                        
Summary Of Significant Accounting Policies [Line Items]                        
Allowance for obsolete expendable parts and supplies                 $ 1,800,000 $ 1,600,000    
Percentage leased                 19.00%      
Aircraft and Rotable Spare Parts [Member]                        
Summary Of Significant Accounting Policies [Line Items]                        
Property and equipment, salvage value, percentage 20.00%               20.00%      
Letter of Credit [Member]                        
Summary Of Significant Accounting Policies [Line Items]                        
Letter of credit facility, amount                 $ 6,000,000