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New Accounting Pronouncements (Details) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 USD ($) aircraft | Mar. 31, 2017 USD ($) | Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Percent | 23.30% |  | 36.90%
Aircraft rentals | $ 40 | $ 54 |
Aircraft under operating lease | aircraft | 53 |  |
Net Present Value Aircraft Operating Leases | $ 900 |  |
ASU 2017-07 |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification | 3 |  |
AOCI Including Portion Attributable to Noncontrolling Interest |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification | $ 20 |  |
B-717-200 |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Aircraft under operating lease | aircraft | 76 |  |
Reclassification out of Accumulated Other Comprehensive Income |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
Reclassification of Stranded amounts in AOCI to Retained Earnings (ASU 2018-02) | 2 |  |

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