Goodwill And Other Intangibles (Narrative) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended			3 Months Ended
Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Sep. 30, 2013 Risk and Insurance Services Segment [Member]	Sep. 30, 2012 Risk and Insurance Services Segment [Member]	Sep. 30, 2013 Consulting Segment [Member]
Segment Reporting Information [Line Items]
Goodwill	$ 6,865	$ 6,679	$ 6,792	$ 6,562	$ 4,700		$ 2,200
Intangible asset impairment	5	8			5	8
Aggregate amortization expense	$ 53	$ 53

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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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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The amount of impairment loss recognized in the period resulting from the write-down of the carrying amount of a finite-lived intangible asset to fair value.

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