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Employee Benefits - Schedule of changes in Level 3 fair value for U.K. Pension Plans (Details) - U.K. - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2016 | Dec. 31, 2015
Actual return on plan assets: |  |
Balance at the beginning of the period | $ 5,903 | $ 6,224
Foreign exchange | (1,127) | (257)
Balance at the end of the period | 5,675 | 5,903
Global |  |
Actual return on plan assets: |  |
Balance at the beginning of the period | 360 |
Balance at the end of the period | 365 | 360
Annuities |  |
Actual return on plan assets: |  |
Balance at the beginning of the period | 827 |
Balance at the end of the period | 1,773 | 827
Alternative investments |  |
Actual return on plan assets: |  |
Balance at the beginning of the period | 717 |
Balance at the end of the period | 773 | 717
Pooled funds - Fixed income securities |  |
Actual return on plan assets: |  |
Balance at the beginning of the period | 283 |
Balance at the end of the period | 223 | 283
Significant Unobservable Inputs (Level 3) |  |
Actual return on plan assets: |  |
Balance at the beginning of the period | 827 |
Balance at the end of the period | 1,773 | 827
Significant Unobservable Inputs (Level 3) | Global |  |
Actual return on plan assets: |  |
Balance at the beginning of the period | 0 |
Balance at the end of the period | 0 | 0
Significant Unobservable Inputs (Level 3) | Annuities |  |
Actual return on plan assets: |  |
Balance at the beginning of the period | 827 | 836
Relating to assets still held at the end of the year | 7 | (32)
Purchases, sales and settlements—net | 1,248 | 58
Foreign exchange | (309) | (35)
Balance at the end of the period | 1,773 | 827
Significant Unobservable Inputs (Level 3) | Alternative investments |  |
Actual return on plan assets: |  |
Balance at the beginning of the period | 0 |
Balance at the end of the period | 0 | 0
Significant Unobservable Inputs (Level 3) | Pooled funds - Fixed income securities |  |
Actual return on plan assets: |  |
Balance at the beginning of the period | 0 |
Balance at the end of the period | $ 0 | $ 0

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- Definition Actual return on plan assets, separately identifying the amount related to assets still held at the reporting date. + 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)(iv)(02)(A) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c)(iv)(02)(A) -URI http://asc.fasb.org/extlink&oid=65897934&loc=d3e2446-114920 + 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 30 -Section 35 -Paragraph 50 -URI http://asc.fasb.org/extlink&oid=64937038&loc=d3e12355-114930 Reference 2: 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=65897934&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Plan Assets -URI http://asc.fasb.org/extlink&oid=6749337 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Plan Assets -URI http://asc.fasb.org/extlink&oid=6520858 + Details
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- Definition The amount of increase or decrease in plan assets attributed to foreign currency changes. The effects of foreign currency exchange rate changes that are to be disclosed are those applicable to plans of a foreign operation whose functional currency is not the reporting currency. + 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=65897934&loc=d3e1928-114920 + Details
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