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Employee Benefits - Schedule of changes in Level 3 fair value for U.K. Pension Plans (Details) - Pension Plan - U.K. - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016
Actual return on plan assets: |  |
Balance at the beginning of the period | $ 5,675 | $ 5,903
Foreign exchange | (513) | 1,127
Balance at the end of the period | 5,906 | 5,675
Annuities |  |
Actual return on plan assets: |  |
Balance at the beginning of the period | 1,773 |
Balance at the end of the period | 1,909 | 1,773
Significant Unobservable Inputs (Level 3) |  |
Actual return on plan assets: |  |
Balance at the beginning of the period | 1,773 |
Balance at the end of the period | 1,909 | 1,773
Significant Unobservable Inputs (Level 3) | Annuities |  |
Actual return on plan assets: |  |
Balance at the beginning of the period | 1,773 | 827
Relating to assets still held at the end of the year | (66) | 7
Purchases, sales and settlements—net | 45 | 1,248
Foreign exchange | 157 | (309)
Balance at the end of the period | $ 1,909 | $ 1,773

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- Definition Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from actual return (loss) on assets still held. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(5)(iv)(02)(A) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount of foreign currency translation gain (loss) which increases (decreases) plan assets of defined benefit plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b)(2) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Amount, measured using unobservable inputs, of increase (decrease) in plan assets of defined benefit plan from purchases, sales and settlements. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(5)(iv)(02)(B) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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