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Pension and Other Postemployment Benefits (Details 5) - OPEB - USD ($) $ in Millions | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value | $ 441 | $ 424
Asset Allocation | 100.00% | 100.00%
Cash and short-term securities |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value | $ 75 | $ 27
Asset Allocation | 17.00% | 6.00%
Domestic |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value | $ 220 | $ 223
Asset Allocation | 50.00% | 53.00%
U.S. treasuries |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value |  | $ 40
Asset Allocation |  | 9.00%
Government bonds |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value | $ 109 | $ 108
Asset Allocation | 24.00% | 26.00%
Corporate bonds |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value | $ 34 | $ 24
Asset Allocation | 8.00% | 6.00%
Mortgaged-backed securities (non-government) |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value | $ 3 | $ 2
Asset Allocation | 1.00% | 0.00%
Level 1 |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value | $ 220 | $ 223
Level 1 | Cash and short-term securities |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value | 0 | 0
Level 1 | Domestic |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value | 220 | 223
Level 1 | U.S. treasuries |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value |  | 0
Level 1 | Government bonds |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value | 0 | 0
Level 1 | Corporate bonds |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value | 0 | 0
Level 1 | Mortgaged-backed securities (non-government) |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value |  | 0
Level 2 |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value | 221 | 201
Level 2 | Cash and short-term securities |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value | 75 | 27
Level 2 | Domestic |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value | 0 | 0
Level 2 | U.S. treasuries |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value |  | 40
Level 2 | Government bonds |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value | 109 | 108
Level 2 | Corporate bonds |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value | 34 | 24
Level 2 | Mortgaged-backed securities (non-government) |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value | 3 | 2
Level 3 |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value | 0 | 0
Level 3 | Cash and short-term securities |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value | 0 | 0
Level 3 | Domestic |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value | 0 | 0
Level 3 | U.S. treasuries |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value |  | 0
Level 3 | Government bonds |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value | 0 | 0
Level 3 | Corporate bonds |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value | 0 | 0
Level 3 | Mortgaged-backed securities (non-government) |  |
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items] |  |
Pension investments measured at fair value | $ 0 | $ 0

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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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