Goodwill and Other Intangible Assets - Schedule of changes in the net carrying amount of goodwill by operating segment (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Changes in the net carrying amount of goodwill by operating segment (in millions)
Balance at the beginning of the period	$ 8,997us-gaap_Goodwill	$ 8,943us-gaap_Goodwill
Goodwill related to acquisitions	292us-gaap_GoodwillAcquiredDuringPeriod	38us-gaap_GoodwillAcquiredDuringPeriod
Goodwill related to disposals	(14)	(12)
Goodwill related to other prior year acquisitions	(8)	15
Transfer	0us-gaap_GoodwillOtherChanges
Foreign currency translation	(407)us-gaap_GoodwillTranslationAdjustments	13us-gaap_GoodwillTranslationAdjustments
Balance at the end of the period	8,860us-gaap_Goodwill	8,997us-gaap_Goodwill
Risk Solutions [Member]
Changes in the net carrying amount of goodwill by operating segment (in millions)
Balance at the beginning of the period	6,020	5,982
Goodwill related to acquisitions	287	36
Goodwill related to disposals	(14)	(9)
Goodwill related to other prior year acquisitions	(8)	(2)
Transfer	(2)
Foreign currency translation	(372)	13
Balance at the end of the period	5,911	6,020
HR Solutions [Member]
Changes in the net carrying amount of goodwill by operating segment (in millions)
Balance at the beginning of the period	2,977	2,961
Goodwill related to acquisitions	5	2
Goodwill related to disposals	0	(3)
Goodwill related to other prior year acquisitions	0	17
Transfer	2
Foreign currency translation	(35)	0
Balance at the end of the period	$ 2,949	$ 2,977

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=14024403&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

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Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

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Amount of other (increase) decrease of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

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Amount of increase (decrease) from adjustments after acquisition date under purchase accounting of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (g)

-URI http://asc.fasb.org/extlink&oid=14024403&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 10

-Section 25

-Paragraph 16

-URI http://asc.fasb.org/extlink&oid=21917927&loc=d3e961-128460

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Amount of increase (decrease) from foreign currency translation adjustments of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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- Definition

Amount of divestiture of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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