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Segment Information - Schedule of consolidated revenue by geographic area (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
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 and Long-Lived Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue | $ 2,909 | $ 2,340 | $ 2,368 | $ 2,381 | $ 2,650 | $ 2,201 | $ 2,282 | $ 2,276 | $ 9,998 | $ 9,409 | $ 9,480
U.S. |  |  |  |  |  |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  |  |  |  | 4,425 | 3,981 | 3,924
Americas other than U.S. |  |  |  |  |  |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  |  |  |  | 976 | 899 | 1,118
U.K. |  |  |  |  |  |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  |  |  |  | 1,436 | 1,354 | 1,419
Europe, Middle East, & Africa |  |  |  |  |  |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  |  |  |  | 2,025 | 1,760 | 1,826
Asia Pacific |  |  |  |  |  |  |  |  |  |  |
Revenues from External Customers and Long-Lived Assets [Line Items] |  |  |  |  |  |  |  |  |  |  |
Total revenue |  |  |  |  |  |  |  |  | $ 1,136 | $ 1,415 | $ 1,193

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