Severance and Leased Real Estate (Details) $ in Millions	9 Months Ended
Sep. 30, 2018 USD ($)
Employee severance
Restructuring reserve
Balance at the beginning of the period	$ 33
Accrued to expense	126
Payments, net	(137)
Balance at the end of the period	22
Leased real estate
Leased Real Estate
Current portion of leased real estate accrual	21
Noncurrent portion of leased real estate accrual	87
Restructuring reserve
Balance at the beginning of the period	64
Accrued to expense	60
Payments, net	(16)
Balance at the end of the period	$ 108
Minimum \| Leased real estate
Leased Real Estate
Lease term	3 months 5 days
Maximum \| Leased real estate
Leased Real Estate
Lease term	12 years 3 months 19 days
Weighted Average \| Leased real estate
Leased Real Estate
Lease term	7 years 2 days

Term of lessee's operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 20
-Section 50
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-URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918673-209980

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Amount of cash payments made as the result of exit or disposal activities. Excludes payments associated with a discontinued operation or an asset retirement obligation.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 420
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)(2)
-URI http://asc.fasb.org/extlink&oid=6394359&loc=d3e17939-110869

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
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Amount of expenses associated with exit or disposal activities pursuant to an authorized plan. Excludes expenses related to a discontinued operation or an asset retirement obligation.

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Carrying amount (including both current and noncurrent portions of the accrual) as of the balance sheet date pertaining to a specified type of cost associated with exit from or disposal of business activities or restructuring pursuant to a duly authorized plan.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Carrying amount as of the balance sheet date of known and estimated costs associated with exit from or disposal of business activities or restructurings pursuant to a duly authorized plan, which are expected to be paid after one year or beyond the next operating cycle, if longer. Costs of such activities include those for one-time termination benefits, termination of an operating lease or other contract, consolidating or closing facilities, and relocating employees, and costs associated with an ongoing benefit arrangement, but excludes costs associated with the retirement of a long-lived asset.

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