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Long-Term Commitments (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Concentration Risk [Line Items] |  |  |
Net rental costs | $ 383 | $ 354 | $ 367
Rentals from subleases | $ 8 | $ 8 | $ 9
Office Building | Lease Concentration Risk | Lease Obligations |  |  |
Concentration Risk [Line Items] |  |  |
Lease obligations for office space | 98.00% |  |

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