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Leases (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Concentration Risk [Line Items] |  |  |
Operating lease, impairment loss | $ 28 | $ 9 |
Lessee, operating lease, lease not yet commenced, amount | $ 3 |  |
Lessee, operating lease, lease not yet commenced, term | 12 months |  |
Net rental costs |  |  | $ 383
Office Space | Lease Concentration Risk | Lease Obligations |  |  |
Concentration Risk [Line Items] |  |  |
Lease obligations for the use of office space (as a percent) | 99.00% |  |
Minimum |  |  |
Concentration Risk [Line Items] |  |  |
Non-cancelable operating leases, term | 10 years |  |
Maximum |  |  |
Concentration Risk [Line Items] |  |  |
Non-cancelable operating leases, term | 25 years |  |

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