Income Taxes (Narrative) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
State and foreign withholding taxes not accrued		$ 2,300,000,000
Effective tax rate		25.60%	42.90%	27.60%
Provisional estimate of the transition tax	$ 240,000,000
Transition tax payment period		8 years
Change in tax rate, deferred tax liability, provisional income tax benefit		$ 220,000,000
Impact of adjustment to estimate tax expense		5,000,000
Increase (decrease) in valuation allowances		36,000,000	$ 9,000,000	$ (8,000,000)
Increase (decrease) in adjustment of valuation allowances, beginning balance		$ 1,000,000	$ 11,000,000	$ (7,000,000)
Net operating loss carryforwards		53.00%
Tax Credit Carryforward [Line Items]
U.S. Federal statutory rate		21.00%	35.00%	35.00%
Unrecognized tax benefits that would impact effective tax rate	56,000,000	$ 64,000,000	$ 56,000,000	$ 53,000,000
Accrued interest and penalties	12,000,000	15,000,000	12,000,000	11,000,000
Putnam and issues included in consolidated MMC tax returns	$ 71,000,000	78,000,000	$ 71,000,000	$ 65,000,000	$ 74,000,000
Federal
Tax Credit Carryforward [Line Items]
Operating loss carryforwards tax benefits		3,000,000
State and Local
Tax Credit Carryforward [Line Items]
Operating loss carryforwards tax benefits		42,000,000
Non-U.S.
Tax Credit Carryforward [Line Items]
Operating loss carryforwards tax benefits		57,000,000
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		$ 10,000,000