Long-Term Commitments (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
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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