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Summary of Significant Accounting Policies - Chase Agreement (Details) - USD ($) $ / shares in Units, $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Basic Earnings Per Share | $ 0.82 | $ 0.47 | $ 0.34
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Net Income | $ 496 | $ 293 | $ 227
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Operating Results | $ 544 | $ 544 | $ 427
New Accounting Pronouncement or Change in Accounting Principle, Effect of Change on Diluted Earnings Per Share | $ 0.82 | $ 0.46 | $ 0.34
Passenger Revenue |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Description | -364 | -250 | -89
Special Revenue Adjustment |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Description | 0 | 0 | 172
Other Revenue |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |
New Accounting Pronouncement or Change in Accounting Principle, Description | 908 | 794 | 344

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