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Basis of Presentation (Details) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended
--- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017
New Accounting Pronouncements and Changes in Accounting Principles [Abstract] |  |  |  |
Aircraft rentals | $ 40 | $ 53 | $ 80 | $ 107

X
- Definition Expenses incurred related to the lease of aircraft from outside third parties that are used in the entity's business operations. + References No definition available. + Details
Name: | us-gaap_AircraftRental
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_NewAccountingPronouncementsAndChangesInAccountingPrinciplesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration