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Summary of Significant Accounting Policies Aircraft Maintenance and Repair Costs (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
A320 Series Engine |  |
Servicing Asset at Amortized Cost [Line Items] |  |
Deferred Costs for Heavy Maintenance | $ 12.8 | $ 64.1
Amortization of Other Deferred Charges | 17.6 | 11.1
Airbus A320 Aircraft Series |  |
Servicing Asset at Amortized Cost [Line Items] |  |
Deferred Costs for Heavy Maintenance | 22.6 | 18.4
Amortization of Other Deferred Charges | $ 19.9 | $ 14.9

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