Summary of Significant Accounting Policies (Narratives) (Details) (USD $) Share data in Millions, unless otherwise specified	1 Months Ended		12 Months Ended												0 Months Ended	12 Months Ended
	Jun. 30, 2010	Dec. 31, 2009	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 Consulting [Member]	Dec. 31, 2010 Consulting [Member]	Dec. 31, 2009 Consulting [Member]	Dec. 31, 2011 Risk and Insurance Services [Member]	Dec. 31, 2010 Risk and Insurance Services [Member]	Dec. 31, 2009 Risk and Insurance Services [Member]	Dec. 31, 2011 Software Development [Member]	Dec. 31, 2011 Private Equity Funds [Member]	Dec. 31, 2010 Private Equity Funds [Member]	Dec. 31, 2009 Private Equity Funds [Member]	Aug. 03, 2010 Kroll Divestiture [Member]	Dec. 31, 2010 Kroll Divestiture [Member]
Cash provided from sale of Kroll															$ 1,130,000,000	$ 1,130,000,000
Interest on fiduciary funds					4,000,000	4,000,000	4,000,000	47,000,000	45,000,000	54,000,000
Fixed income securities in fiduciary assets			62,000,000	283,000,000
Unrealized gains, net of tax			2,000,000	5,000,000
Net uncollected premiums and claims receivable and payable			9,000,000,000	9,100,000,000
Performance fee revenue			74,000,000
Investment securities												10,000,000	32,000,000	(6,000,000)
Equity method investments												78,000,000
Purchase commitment, remaining minimum amount committed												67,000,000
Indefinite-lived intangible assets			0	0
Capitalized software costs amortized period minimum, years											3
Capitalized software costs amortized period maximum, years											10
Capitalized computer software costs			244,000,000	215,000,000
Capitalized computer software costs net of accumulated amortization			619,000,000	572,000,000
Tax position, amount recognized, min			50.00%
Stock options outstanding		46.4	38.9	43.4
Amounts paid under terms of settlement agreement	500,000,000
Amount covered by insurance	100,000,000	205,000,000
Amount agreed to pay without admitting liability		425,000,000
Settlement agreement, recognized charge after insurance	400,000,000	220,000,000
Assets in trusts or funds for management or trustee fee			$ 15,000,000,000