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Summary of Significant Accounting Policies - Additional Information (Detail) | 3 Months Ended | 12 Months Ended
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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 |  |  |

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