Summary of Significant Accounting Policies (Narratives) (Details)	12 Months Ended
	Dec. 31, 2016 USD ($) segment shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($) shares
Schedule of Equity Method Investments [Line Items]
Number of business segments | segment	2
Uncollected premiums and claims, net of related payables	$ 7,000,000,000	$ 6,900,000,000
Restricted cash and cash equivalents	159,000,000
Investment income (less than)	1,000,000	38,000,000	$ 37,000,000
Indefinite-lived intangible assets	0	0
Capitalized computer software costs	482,000,000	498,000,000
Capitalized computer software costs accumulated amortization	$ 1,100,000,000	$ 958,000,000
Stock options outstanding (in shares) | shares	13,242,529	14,776,804	18,000,000
Mercer Consulting Group
Schedule of Equity Method Investments [Line Items]
Assets in trusts or funds for management or trustee fee	$ 158,000,000,000
Minimum | Software Development
Schedule of Equity Method Investments [Line Items]
Property, plant and equipment, useful life	3 years
Maximum | Software Development
Schedule of Equity Method Investments [Line Items]
Property, plant and equipment, useful life	10 years
Risk and Insurance Services Segment
Schedule of Equity Method Investments [Line Items]
Interest income, fiduciary assets	$ 26,000,000	$ 21,000,000	$ 24,000,000
Consulting Segment
Schedule of Equity Method Investments [Line Items]
Interest income, fiduciary assets	$ 3,000,000	$ 4,000,000	$ 6,000,000
Customer Lists | Minimum
Schedule of Equity Method Investments [Line Items]
Intangible asset useful life	10 years
Customer Lists | Maximum
Schedule of Equity Method Investments [Line Items]
Intangible asset useful life	15 years