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Employee Benefits - Additional Information (Details) £ in Millions, $ in Millions | 3 Months Ended | 9 Months Ended |
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Sep. 30, 2018 GBP (£) | Sep. 30, 2018 USD ($) | Sep. 30, 2018 GBP (£) | Sep. 30, 2018 USD ($) | Dec. 31, 2018 USD ($)
UNITED STATES |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Estimate of contributions to defined benefit pension plans for the current fiscal year |  |  |  |  | $ 46
U.K. |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Payment for settlement |  |  | £ 125 | $ 164 |
Benefit obligation period increase (decrease) | £ (108) | $ (143) |  |  |
Benefit obligation, (increase) decrease for settlement | £ 7 | $ 9 | £ 24 | $ 32 |
Estimate of contributions to defined benefit pension plans for the current fiscal year |  |  |  |  | 80
Other |  |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |
Estimate of contributions to defined benefit pension plans for the current fiscal year |  |  |  |  | $ 19

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