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Employee Benefits - Reconciliation of the changes in the benefit obligations and fair value of assets and a statement of the funded status (Details) - Pension Plan £ in Millions, $ in Millions | 12 Months Ended
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Dec. 31, 2019 USD ($) | Dec. 31, 2018 GBP (£) | Dec. 31, 2018 USD ($) | Dec. 31, 2017 GBP (£) | Dec. 31, 2017 USD ($)
U.S. |  |  |  |  |
Change in projected benefit obligation |  |  |  |  |
Balance at the beginning of the Period | $ 2,877 |  | $ 3,155 |  |
Service cost | 0 |  | 0 |  | $ 0
Interest cost | 108 |  | 99 |  | 96
Plan amendment | 0 |  | 0 |  |
Settlements | 0 |  | 0 |  |
Actuarial loss (gain) | 373 |  | (221) |  |
Benefit payments | (166) |  | (156) |  |
Foreign currency impact | 0 |  | 0 |  |
Balance at the end of the period | 3,192 |  | 2,877 |  | 3,155
Accumulated benefit obligation at end of year | 3,192 |  | 2,877 |  |
Change in fair value of plan assets |  |  |  |  |
Balance at the beginning of the period | 1,796 |  | 1,958 |  |
Actual return on plan assets | 398 |  | (141) |  |
Employer contributions | 38 |  | 135 |  |
Settlements | 0 |  | 0 |  |
Benefit payments | (166) |  | (156) |  |
Foreign currency impact | 0 |  | 0 |  |
Balance at the end of the period | 2,066 |  | 1,796 |  | 1,958
Market related value at end of year | 1,969 |  | 1,981 |  |
Funded status | (1,126) |  | (1,081) |  |
Unrecognized prior-service cost | 1 |  | 3 |  |
Unrecognized loss | 1,762 |  | 1,705 |  |
Net amount recognized | 637 |  | 627 |  |
U.K. |  |  |  |  |
Change in projected benefit obligation |  |  |  |  |
Balance at the beginning of the Period | 4,129 |  | 4,893 |  |
Service cost | 0 |  | 0 |  | 0
Interest cost | 109 |  | 109 |  | 123
Plan amendment | 10 |  | 13 |  |
Settlements | (22) |  | (176) |  |
Actuarial loss (gain) | 594 |  | (297) |  |
Benefit payments | (168) |  | (160) |  |
Foreign currency impact | 127 |  | (253) |  |
Balance at the end of the period | 4,779 |  | 4,129 |  | 4,893
Accumulated benefit obligation at end of year | 4,779 |  | 4,129 |  |
Change in fair value of plan assets |  |  |  |  |
Balance at the beginning of the period | 5,225 |  | 5,906 |  |
Actual return on plan assets | 687 |  | (125) |  |
Employer contributions | 78 |  | 97 |  |
Settlements | (22) | £ (139) | (176) | £ (371) | (496)
Benefit payments | (168) |  | (160) |  |
Foreign currency impact | 159 |  | (317) |  |
Balance at the end of the period | 5,959 |  | 5,225 |  | 5,906
Market related value at end of year | 5,959 |  | 5,225 |  |
Funded status | 1,180 |  | 1,096 |  |
Unrecognized prior-service cost | 40 |  | 30 |  |
Unrecognized loss | 1,204 |  | 1,106 |  |
Net amount recognized | 2,424 |  | 2,232 |  |
Other |  |  |  |  |
Change in projected benefit obligation |  |  |  |  |
Balance at the beginning of the Period | 1,271 |  | 1,401 |  |
Service cost | 0 |  | 0 |  | 0
Interest cost | 27 |  | 27 |  | 26
Plan amendment | 0 |  | 0 |  |
Settlements | 0 |  | 0 |  |
Actuarial loss (gain) | 177 |  | (47) |  |
Benefit payments | (42) |  | (43) |  |
Foreign currency impact | (8) |  | (67) |  |
Balance at the end of the period | 1,425 |  | 1,271 |  | 1,401
Accumulated benefit obligation at end of year | 1,391 |  | 1,247 |  |
Change in fair value of plan assets |  |  |  |  |
Balance at the beginning of the period | 1,155 |  | 1,256 |  |
Actual return on plan assets | 182 |  | (19) |  |
Employer contributions | 19 |  | 20 |  |
Settlements | 0 |  | 0 |  |
Benefit payments | (42) |  | (43) |  |
Foreign currency impact | (11) |  | (59) |  |
Balance at the end of the period | 1,303 |  | 1,155 |  | $ 1,256
Market related value at end of year | 1,303 |  | 1,155 |  |
Funded status | (122) |  | (116) |  |
Unrecognized prior-service cost | (6) |  | (7) |  |
Unrecognized loss | 460 |  | 440 |  |
Net amount recognized | $ 332 |  | $ 317 |  |

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- Definition Amount, before tax, of accumulated other comprehensive income (loss) for gain (loss) of defined benefit plan, that has not been recognized in net periodic benefit (cost) credit. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (j) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=118930883&loc=SL7669686-108580 + Details
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