Income Taxes (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Disclosure [Abstract]
Investments in foreign subsidiaries	$ 4,400,000,000
Effective tax rate	27.60%	29.10%	28.50%
Valuation allowances	$ (8,000,000)	$ (69,000,000)	$ 15,000,000
Valuation allowances, beginning balance	$ (7,000,000)	(14,000,000)	(9,000,000)
Net operating loss carryforwards	77.00%
Tax Credit Carryforward [Line Items]
Unrecognized tax benefits that would impact effective tax rate	$ 53,000,000	53,000,000	51,000,000
Accrued interest and penalties	11,000,000	8,000,000	7,000,000
Putnam and issues included in consolidated MMC tax returns	65,000,000	74,000,000	97,000,000	$ 128,000,000
Federal
Tax Credit Carryforward [Line Items]
Operating loss carryforwards tax benefits	7,000,000
State and Local
Tax Credit Carryforward [Line Items]
Operating loss carryforwards tax benefits	48,000,000
Non-U.S.
Tax Credit Carryforward [Line Items]
Operating loss carryforwards tax benefits	31,000,000
Putnam and Kroll
Tax Credit Carryforward [Line Items]
Putnam and issues included in consolidated MMC tax returns	0	$ 1,000,000	$ 2,000,000
Minimum
Tax Credit Carryforward [Line Items]
Decrease in unrecognized tax benefits is reasonably possible	0
Maximum
Tax Credit Carryforward [Line Items]
Decrease in unrecognized tax benefits is reasonably possible	$ 7,000,000