The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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The amount of goodwill arising from a business combination that is expected to be deductible for tax purposes.

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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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Goodwill and Other Intangibles (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Goodwill acquired	$ 410	$ 533
Goodwill	9,435	9,100	$ 9,089	$ 8,369
Aggregate amortization expense	135	$ 122
Risk and Insurance Services Segment
Segment Reporting Information [Line Items]
Goodwill acquired	368
Business acquisition, goodwill, expected tax deductible amount	308
Goodwill	6,800
Consulting Segment
Segment Reporting Information [Line Items]
Goodwill acquired	42
Goodwill	$ 2,600