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Benefit Plans - Weighted average target allocation and actual percentage (Details) - Pension benefits - USD ($) $ in Millions | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | --- | ---
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 | $ 5,299 | $ 6,284 | $ 5,650
Equity securities |  |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |  |
Actual percentage of plan assets | 47.00% | 58.00% |
Equity securities | Minimum |  |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |  |
Target asset allocation | 36.00% |  |
Equity securities | Maximum |  |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |  |
Target asset allocation | 53.00% |  |
Fixed income securities |  |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |  |
Actual percentage of plan assets | 41.00% | 34.00% |
Fixed income securities | Minimum |  |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |  |
Target asset allocation | 40.00% |  |
Fixed income securities | Maximum |  |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |  |
Target asset allocation | 53.00% |  |
Limited partnership interests |  |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |  |
Actual percentage of plan assets | 9.00% | 6.00% |
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% |  |
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% |  |
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 | 3.00% | 2.00% |
Private equity funds |  |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |  |
Actual percentage of plan assets | 1.00% | 2.00% |
Mutual funds |  |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |  |
Target asset allocation | 4.00% | 3.00% |
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 | $ 208 | $ 202 |

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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=108410482&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=108410482&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=108410482&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=112275985&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 (b) -URI http://asc.fasb.org/extlink&oid=108410482&loc=d3e1928-114920 Reference 6: 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=108410482&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=112275985&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=108410482&loc=d3e1928-114920 + Details
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