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Basis of Presentation (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2018 | Mar. 31, 2017
New Accounting Pronouncements and Changes in Accounting Principles [Abstract] |  |
Aircraft rentals | $ 40 | $ 54

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

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