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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
--- | ---
Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Dec. 31, 2018 GBP (£)
U.S. |  |  |  |
Change in projected benefit obligation |  |  |  |
Balance at the beginning of the Period | $ 3,192 | $ 2,877 |  |
Service cost | 0 | 0 | $ 0 |
Interest cost | 85 | 108 | 99 |
Plan amendment | 0 | 0 |  |
Settlements | 0 | 0 |  |
Actuarial loss (gain) | 274 | 373 |  |
Benefit payments | (171) | (166) |  |
Foreign currency impact | 0 | 0 |  |
Balance at the end of the period | 3,380 | 3,192 | 2,877 |
Accumulated benefit obligation at end of year | 3,380 | 3,192 |  |
Change in fair value of plan assets |  |  |  |
Balance at the beginning of the period | 2,066 | 1,796 |  |
Actual return on plan assets | 289 | 398 |  |
Employer contributions | 92 | 38 |  |
Settlements | 0 | 0 |  |
Benefit payments | (171) | (166) |  |
Foreign currency impact | 0 | 0 |  |
Balance at the end of the period | 2,276 | 2,066 | 1,796 |
Market related value at end of year | 2,076 | 1,969 |  |
Funded status | (1,104) | (1,126) |  |
Unrecognized prior-service cost | 0 | 1 |  |
Unrecognized loss | 1,812 | 1,762 |  |
Net amount recognized | 708 | 637 |  |
U.K. |  |  |  |
Change in projected benefit obligation |  |  |  |
Balance at the beginning of the Period | 4,779 | 4,129 |  |
Service cost | 0 | 0 | 0 |
Interest cost | 88 | 109 | 109 |
Plan amendment | 3 | 10 |  |
Settlements | (7) | (22) |  |
Actuarial loss (gain) | 520 | 594 |  |
Benefit payments | (209) | (168) |  |
Foreign currency impact | 232 | 127 |  |
Balance at the end of the period | 5,406 | 4,779 | 4,129 |
Accumulated benefit obligation at end of year | 5,406 | 4,779 |  |
Change in fair value of plan assets |  |  |  |
Balance at the beginning of the period | 5,959 | 5,225 |  |
Actual return on plan assets | 618 | 687 |  |
Employer contributions | 8 | 78 |  |
Settlements | (7) | (22) | (176) | £ (139)
Benefit payments | (209) | (168) |  |
Foreign currency impact | 283 | 159 |  |
Balance at the end of the period | 6,652 | 5,959 | 5,225 |
Market related value at end of year | 6,652 | 5,959 |  |
Funded status | 1,246 | 1,180 |  |
Unrecognized prior-service cost | 43 | 40 |  |
Unrecognized loss | 1,286 | 1,204 |  |
Net amount recognized | 2,575 | 2,424 |  |
Other |  |  |  |
Change in projected benefit obligation |  |  |  |
Balance at the beginning of the Period | 1,425 | 1,271 |  |
Service cost | 0 | 0 | 0 |
Interest cost | 19 | 27 | 27 |
Plan amendment | 0 | 0 |  |
Settlements | 0 | 0 |  |
Actuarial loss (gain) | 112 | 177 |  |
Benefit payments | (44) | (42) |  |
Foreign currency impact | 113 | (8) |  |
Balance at the end of the period | 1,625 | 1,425 | 1,271 |
Accumulated benefit obligation at end of year | 1,592 | 1,391 |  |
Change in fair value of plan assets |  |  |  |
Balance at the beginning of the period | 1,303 | 1,155 |  |
Actual return on plan assets | 109 | 182 |  |
Employer contributions | 20 | 19 |  |
Settlements | 0 | 0 |  |
Benefit payments | (44) | (42) |  |
Foreign currency impact | 109 | (11) |  |
Balance at the end of the period | 1,497 | 1,303 | $ 1,155 |
Market related value at end of year | 1,497 | 1,303 |  |
Funded status | (128) | (122) |  |
Unrecognized prior-service cost | (7) | (6) |  |
Unrecognized loss | 521 | 460 |  |
Net amount recognized | $ 386 | $ 332 |  |

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- Definition Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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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://www.xbrl.org/2003/role/disclosureRef -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=121641772&loc=SL7669686-108580 + Details
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- Definition Amount, before tax, of accumulated other comprehensive (income) loss for cost (credit) of benefit change attributable to participants' prior service from plan amendment or plan initiation of defined benefit plan, that has not been recognized in net periodic benefit cost (credit). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -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=121641772&loc=SL7669686-108580 + Details
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