Restructuring Costs (Restructuring Activities) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Restructuring Reserve [Roll Forward]
Liability at beginning of period	$ 170	$ 181
Amounts Accrued	17	78
Cash Paid	(58)	(88)
Other	1	(1)
Liability at end of period	130	170
Severance [Member]
Restructuring Reserve [Roll Forward]
Liability at beginning of period	36	27
Amounts Accrued	8	46
Cash Paid	(30)	(38)
Other	(2)	1
Liability at end of period	12	36
Future Rent Under Non-Cancelable Leases and Other Costs [Member]
Restructuring Reserve [Roll Forward]
Liability at beginning of period	134	154
Amounts Accrued	9	32
Cash Paid	(28)	(50)
Other	3	(2)
Liability at end of period	118	134
Risk and Insurance Services Segment [Member]
Restructuring Reserve [Roll Forward]
Amounts Accrued	5
Consulting Segment [Member]
Restructuring Reserve [Roll Forward]
Amounts Accrued	1
Corporate Segment [Member]
Restructuring Reserve [Roll Forward]
Amounts Accrued	$ 11