Summary of Significant Accounting Policies - Prospective Adoption of New Accounting Pronouncements (Details) - Accounting Standards Update 2017-07 - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Item Effected [Line Items]
Increase in Compensation and Benefits	$ 201	$ 233
Other Net Periodic Benefit Credit	(201)	(233)
Net Impact of Reclassification	0	0
Risk and Insurance Services Segment
Item Effected [Line Items]
Increase in Compensation and Benefits	140	172
Consulting Segment
Item Effected [Line Items]
Increase in Compensation and Benefits	64	65
Corporate, Non-Segment
Item Effected [Line Items]
Increase in Compensation and Benefits	$ (3)	$ (4)

Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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