Expenses - Restructuring Reserve (Details) - USD ($) $ in Millions	3 Months Ended
Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019
Restructuring Reserve [Roll Forward]
Beginning balance	$ 138	$ 164	$ 198	$ 209	$ 259	$ 341
Accruals for Beacon					2	(4)
Accruals for Repositioning Charges						0
Payments and Other Adjustments	(10)	(26)	(34)	(64)	(52)	(78)
Ending balance	128	138	164	145	209	259
Employee Related Costs
Restructuring Reserve [Roll Forward]
Beginning balance	132	157	190	197	246	303
Accruals for Beacon					2	(4)
Accruals for Repositioning Charges						0
Payments and Other Adjustments	(10)	(25)	(33)	(62)	(51)	(53)
Ending balance	122	132	157	135	197	246
Real Estate Actions
Restructuring Reserve [Roll Forward]
Beginning balance	5	6	7	11	12	37
Accruals for Beacon					0	0
Accruals for Repositioning Charges						0
Payments and Other Adjustments	1	(1)	(1)	(2)	(1)	(25)
Ending balance	6	5	6	9	11	12
Asset and Other Write-offs
Restructuring Reserve [Roll Forward]
Beginning balance	1	1	1	1	1	1
Accruals for Beacon					0	0
Accruals for Repositioning Charges						0
Payments and Other Adjustments	(1)	0	0	0	0	0
Ending balance	$ 0	$ 1	$ 1	$ 1	$ 1	$ 1

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.

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

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
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-Section 45
-Paragraph 17
-Subparagraph (f)
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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.

+ References

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-Publisher FASB
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- Definition

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
-Publisher FASB
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-Topic 420
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- Definition

Amount of expense (reversal of expense) which increases (decreases) the restructuring reserve from an adjustment to a previously accrued restructuring liability.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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