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Goodwill and Other Intangibles (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Goodwill acquired			$ 338	$ 466
Goodwill, expected tax deductible amount	$ 1	$ 141	1	141
Goodwill	15,775	14,797	15,775	14,797	$ 15,517	$ 14,671
Aggregate amortization expense	189	$ 174	189	$ 174
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Goodwill	11,900		11,900
Consulting Segment
Segment Reporting Information [Line Items]
Goodwill	$ 3,900		$ 3,900