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Segment Information - Schedule of Revenue by Geographical Area (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
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Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Revenues from External Customers [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenues | $ 5,949 | $ 6,512 | $ 7,141 | $ 6,505 | $ 7,320 | $ 7,500 | $ 7,776 | $ 7,794 | $ 26,107 | $ 30,390 | $ 32,639
United States [Member] |  |  |  |  |  |  |  |  |  |  |
Revenues from External Customers [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 18,194 | 19,354 | 21,234
Europe [Member] |  |  |  |  |  |  |  |  |  |  |
Revenues from External Customers [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | 5,311 | 6,365 | 7,528
Other Countries [Member] |  |  |  |  |  |  |  |  |  |  |
Revenues from External Customers [Line Items] |  |  |  |  |  |  |  |  |  |  |
Revenues |  |  |  |  |  |  |  |  | $ 2,602 | $ 4,671 | $ 3,877

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