Aircraft Rent Commitments [Abstract]

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Face/par value in contract for future debt financing commitments

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Long-Term Debt, Maturities, Repayments Of Interest And Fee Commitments [Abstract]

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Long-Term Debt, Maturities, Repayments Of Interest And Fee Commitments, After Year Five

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Long-Term Debt, Maturities, Repayments Of Interest And Fee Commitments, Year Five

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Long-Term Debt, Maturities, Repayments Of Interest And Fee Commitments, Year Four

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Long-Term Debt, Maturities, Repayments Of Interest And Fee Commitments, Year Three

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Long-Term Debt, Maturities, Repayments Of Interest And Fee Commitments, Year Two

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Principal And Interest Commitments Related to Future Secured Debt After Year Five

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Principal And Interest Commitments Related to Future Secured Debt, Due In Next Fiscal Year

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Principal And Interest Commitments Related to Future Secured Debt In Year Five

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Principal And Interest Commitments Related to Future Secured Debt In Year Four

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Principal And Interest Commitments Related to Future Secured Debt In Year Three

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Property, Plant and Equipment Purchased, Consideration Transferred

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Amount equal to the present value (the principal) at the beginning of the lease term of minimum lease payments during the lease term (excluding that portion of the payments representing executory costs such as insurance, maintenance, and taxes to be paid by the lessor, together with any profit thereon) net of payments or other amounts applied to the principal through the balance sheet date.

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Gross amount of long-lived, depreciable flight assets used in the entity's principle business operations, subject to a lease and meeting the criteria for capitalization.

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Amount of long-term debt payable, sinking fund requirements, and other securities issued that are redeemable by holder at fixed or determinable prices and dates maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of required minimum rental payments for operating leases having an initial or remaining non-cancelable lease term in excess of one year due after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of the fixed and determinable portion of the unrecorded unconditional purchase obligation maturing after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Commitments and Contingencies - Aircraft-Related Commitments and Financing Arrangements (Details) - USD ($) $ in Millions	3 Months Ended
	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2018	Nov. 30, 2017
Unrecorded Unconditional Purchase Obligation [Line Items]
Capital Leases, Lessee Balance Sheet, Assets by Major Class, Flight Equipment, Gross		$ 236.7
Capital Lease Obligations		143.8
Interest Commitments
Interest commitments, 2019			$ 171.3
Secured Debt
Interest Commitments
Interest commitments, 2019			83.3
Interest commitments, 2020			76.2
Interest commitments, 2021			69.2
Interest commitments, 2022			62.4
Interest commitments, 2023			52.3
Interest commitments, 2024 and beyond			150.3
Principal and Interest Commitments
Principal and interest commitments, 2019			5.7
Principal and interest commitments, 2020			9.4
Principal and interest commitments, 2021			8.1
Principal and interest commitments, 2022			7.1
Principal and interest commitments, 2023 and beyond			52.4
Enhanced Equipment Trust Certificate
Committed Expenditures
Face amount, commitment for future issuance				$ 420.5
Aircraft and Related Flight Equipment
Committed Expenditures
Committed expenditures, 2019			583.8
Committed expenditures, 2020			811.4
Committed expenditures, 2021			775.6
Committed expenditures, 2022			17.7
Committed expenditures, 2023			8.4
Committed expenditures, 2024 and beyond			0.0
Face amount, commitment for future issuance			70.0
Non-aircraft Related Commitments
Committed Expenditures
Committed expenditures, 2019			14.4
Committed expenditures, 2020			14.3
Committed expenditures, 2021			11.3
Committed expenditures, 2022			11.2
Committed expenditures, 2023			11.3
Committed expenditures, 2024 and beyond			56.0
A320NEO \| Third Party Lessor
Aircraft Rent Commitments [Abstract]
Aircraft rent commitments, 2019			18.2
Aircraft rent commitments, 2020			20.2
Aircraft rent commitments, 2021			20.2
Aircraft rent commitments, 2022			20.2
Aircraft rent commitments, 2023			20.2
Aircraft rent commitments, 2024 and thereafter			$ 143.3
Airbus A319
Unrecorded Unconditional Purchase Obligation [Line Items]
Property, Plant and Equipment Purchased, Consideration Transferred	$ 285.0	$ 285.0