Restructuring Costs (Restructuring Activities) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Restructuring Reserve [Roll Forward]
Liability at beginning of period	$ 93	$ 92
Amounts Accrued	3	28
Cash Paid	(10)	(28)
Other	(1)	1
Liability at end of period	85	93
Severance
Restructuring Reserve [Roll Forward]
Liability at beginning of period	15	7
Amounts Accrued	2	17
Cash Paid	(5)	(7)
Other	0	(2)
Liability at end of period	12	15
Future Rent Under Non-Cancelable Leases and Other Costs
Restructuring Reserve [Roll Forward]
Liability at beginning of period	78	85
Amounts Accrued	1	11
Cash Paid	(5)	(21)
Other	(1)	3
Liability at end of period	73	$ 78
Operating Segments | Risk and Insurance Services Segment | Acquisition Related
Restructuring Reserve [Roll Forward]
Amounts Accrued	1
Corporate, Non-Segment | Acquisition Related
Restructuring Reserve [Roll Forward]
Amounts Accrued	$ 2