Acquisitions (Narrative) (Details) $ in Millions		3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 25, 2018 USD ($) colleague	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) acquisition	Jun. 30, 2017 USD ($)	Dec. 31, 2017 acquisition
Business Acquisition [Line Items]
Total consideration				$ 192	$ 510
Cash				160	420
Estimated fair value of deferred/contingent consideration				32	90
Contingent consideration from prior year's acquisitions				32	60
Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Total consideration				192
Cash				160
Estimated fair value of deferred/contingent consideration				32
Revenue related to acquisitions		$ 11	$ 34	14	63
Operating (loss) income related to acquisitions		$ (1)	$ 5	(2)	15
Prior Fiscal Periods Acquisitions
Business Acquisition [Line Items]
Deferred purchase consideration from prior years' acquisitions				53	37
Contingent consideration from prior year's acquisitions				$ 41	$ 65
Wortham Insurance
Business Acquisition [Line Items]
Revenue	$ 130
Number of colleagues | colleague	530
Risk and Insurance Services Segment
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition				7		7
Consulting Segment
Business Acquisition [Line Items]
Number of acquisitions made (in acquisitions) | acquisition				5		3
Minimum
Business Acquisition [Line Items]
Revenue target period (in years)				2 years
Minimum | Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Revenue target period (in years)				2 years
Maximum
Business Acquisition [Line Items]
Revenue target period (in years)				4 years
Maximum | Current Fiscal Period Acquisitions
Business Acquisition [Line Items]
Revenue target period (in years)				4 years