Acquisitions and Dispositions - Acquisitions (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015 business	Mar. 31, 2014 business	Dec. 31, 2014
Intangible assets:
Goodwill	$ 8,548us-gaap_Goodwill		$ 8,860us-gaap_Goodwill
Business Acquisition 1 [Member]
Business Acquisition
Consideration	21	5
Intangible assets:
Goodwill	16	4
Other intangible assets	1	4
Total	17	8
Risk Solutions [Member]
Business Acquisition
Number of business acquired under business combination	1	1
Intangible assets:
Goodwill	5,628		5,911
HR Solutions [Member]
Business Acquisition
Number of business acquired under business combination	1	1
Intangible assets:
Goodwill	$ 2,920		$ 2,949

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Amount of intangible assets, excluding goodwill, acquired at the acquisition date.

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Amount recognized as of the acquisition date for the identifiable assets acquired in excess of (less than) the aggregate liabilities assumed.

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Amount of assets expected to be realized or consumed after one year or the normal operating cycle, if longer, acquired at the acquisition date.

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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.

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