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Long-Term Commitments (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Concentration Risk [Line Items] |  |  |
Net rental costs | $ 354 | $ 367 | $ 381
Rentals from subleases | $ 8 | $ 9 | $ 14
Office Building | Lease Concentration Risk | Lease Obligations |  |  |
Concentration Risk [Line Items] |  |  |
Lease obligations for office space | 99.00% |  |

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- Definition Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 840 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=77902758&loc=d3e41499-112717 + Details
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