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Benefit Plans - Weighted average target allocation and actual percentage (Details) - Pension benefits - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
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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%
Equity securities |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |
Actual percentage of plan assets | 58.00% | 62.00%
Equity securities | Minimum |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |
Target asset allocation | 43.00% |
Equity securities | Maximum |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |
Target asset allocation | 62.00% |
Fixed income securities |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |
Actual percentage of plan assets | 34.00% | 29.00%
Fixed income securities | Minimum |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |
Target asset allocation | 34.00% |
Fixed income securities | Maximum |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |
Target asset allocation | 44.00% |
Derivatives |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |
Actual percentage of plan assets | 1.00% |
Securities pledged as collateral |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |
Assets for plan benefits | $ 202 | $ 143
Limited partnership interests |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |
Actual percentage of plan assets | 6.00% | 7.00%
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 | 13.00% |
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 | 2.00% | 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 | 2.00% | 1.00%
Mutual funds |  |
Pension plans' weighted average target percentage of plan assets and the actual percentage of plan assets |  |
Target asset allocation | 3.00% | 3.00%

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- Definition Amount of asset, recognized in statement of financial position, for overfunded defined benefit pension and other postretirement plans. + 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 (c) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=64852052&loc=d3e1731-114919 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(17)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=64852052&loc=d3e1703-114919 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(10)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(10)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + 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/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(5)(i) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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- Definition Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 5 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2446-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920 + Details
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