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Employee Postretirement Benefits Employee Postretirement Benefits (Pension Plans with a PBO in Excess of Plan Assets) (Details) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
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PBO | $ 744 | $ 2,303
Fair value of plan assets | $ 529 | $ 2,039

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- Definition Amount of plan asset for defined benefit pension plan with projected benefit obligation in excess of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2417-114920 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Amount of projected benefit obligation for defined benefit pension plan with projected benefit obligation in excess of plan assets. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2417-114920 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant