Long-Term Commitments (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Concentration Risk [Line Items]
Net rental costs	$ 367	$ 381	$ 393
Rentals from subleases	$ 9	$ 14	$ 12
Office Building \| Lease Concentration Risk \| Lease Obligations
Concentration Risk [Line Items]
Lease obligations for office space	98.00%

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

+ References

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Rental expense for the reporting period incurred under operating leases, including minimum and any contingent rent expense, net of related sublease income.

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The total amount of sublease rental income recognized during the period that reduces the entity's rent expense incurred under operating leases.

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