Acquisitions and Dispositions of Businesses - Dispositions (Details) $ in Millions	12 Months Ended
Dec. 31, 2016 USD ($) disposal	Dec. 31, 2015 USD ($) disposal	Dec. 31, 2014 USD ($) disposal
Dispositions
Net gain on disposals of businesses \| $	$ 39	$ 82	$ 24
Disposal Group, Not Discontinued Operations
Dispositions
Number of dispositions	5	7	2
Disposal Group, Not Discontinued Operations \| Risk Solutions
Dispositions
Number of dispositions	4	4	2
Disposal Group, Not Discontinued Operations \| HR Solutions
Dispositions
Number of dispositions	1	3	0

Amount before tax of gain (loss) recognized on the sale or disposal of a disposal group. Excludes discontinued operations.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=68055307&loc=d3e2443-110228

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=64862545&loc=d3e2941-110230

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration