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Benefit Plans - Weighted average target allocation and actual percentage (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | --- | ---
Private equity funds |  |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |  |
Actual percentage of plan assets | 0.00% | 1.00% |
Mutual funds |  |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |  |
Target asset allocation | 1.00% | 0.00% |
Pension benefits |  |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |  |
Actual percentage of plan assets | 100.00% | 100.00% |
Fair value of plan assets | $ 6,987 | $ 6,192 | $ 5,299
Pension benefits | Equity securities |  |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |  |
Actual percentage of plan assets | 50.00% | 50.00% |
Pension benefits | Equity securities | Minimum |  |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |  |
Target asset allocation | 43.00% |  |
Pension benefits | Equity securities | Maximum |  |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |  |
Target asset allocation | 62.00% |  |
Pension benefits | Fixed income securities |  |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |  |
Actual percentage of plan assets | 38.00% | 38.00% |
Pension benefits | Fixed income securities | Minimum |  |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |  |
Target asset allocation | 33.00% |  |
Pension benefits | Fixed income securities | Maximum |  |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |  |
Target asset allocation | 45.00% |  |
Pension benefits | Limited partnership interests |  |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |  |
Actual percentage of plan assets | 10.00% | 10.00% |
Pension benefits | Limited partnership interests | Minimum |  |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |  |
Target asset allocation | 0.00% |  |
Pension benefits | Limited partnership interests | Maximum |  |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |  |
Target asset allocation | 15.00% |  |
Pension benefits | Short-term investments and other |  |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |  |
Target asset allocation | 0.00% |  |
Actual percentage of plan assets | 2.00% | 2.00% |
Pension benefits | Securities pledged as collateral |  |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |  |
Fair value of plan assets | $ 101 | $ 258 |

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- Definition Amount of asset segregated and restricted to provide benefit under defined benefit plan. Asset includes, but is not limited to, stock, bond, other investment, earning from investment, and contribution by employer and employee. + 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 (d)(iv)(01) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(01) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(iv)(02) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 Reference 7: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(ii) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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- Definition Percentage of target investment allocation to total plan assets. Includes, but is not limited to, percentage on weighted-average basis if more than one plan. + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -URI http://asc.fasb.org/extlink&oid=118257860&loc=d3e4179-114921 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(i) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920 + Details
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