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Income Taxes (Narrative) (Details) - USD ($) |  | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Jan. 01, 2018 | Dec. 31, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Income Tax Disclosure [Abstract] |  |  |  |  |  |
Effective tax rate |  |  | 42.90% | 27.60% | 29.10% |
Provisional estimate of the transition tax |  | $ 240,000,000 |  |  |  |
Tax Cuts and Jobs Act of 2017, transition tax for accumulated foreign earnings, liability |  | 3,400,000,000 | $ 3,400,000,000 |  |  |
Withholding and state income tax expense (benefit) |  | 53,000,000 |  |  |  |
Tax Cuts and Jobs Act of 2017, income tax expense (benefit) |  |  | 3,400,000,000 |  |  |
Charge to reduce value to deferred tax assets and liabilities |  |  | 220,000,000 |  |  |
Increase (decrease) in valuation allowances |  |  | 9,000,000 | $ 8,000,000 | $ 69,000,000 |
Increase (decrease) in adjustment of valuation allowances, beginning balance |  |  | $ 11,000,000 | $ (7,000,000) | $ (14,000,000) |
Net operating loss carryforwards |  |  | 81.00% |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |  |  |
U.S. Federal statutory rate |  |  | 35.00% | 35.00% | 35.00% |
Unrecognized tax benefits that would impact effective tax rate |  | 56,000,000 | $ 56,000,000 | $ 53,000,000 | $ 53,000,000 |
Accrued interest and penalties |  | 12,000,000 | 12,000,000 | 11,000,000 | 8,000,000 |
Putnam and issues included in consolidated MMC tax returns |  | 71,000,000 | 71,000,000 | $ 65,000,000 | $ 74,000,000 | $ 97,000,000
Federal |  |  |  |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |  |  |
Operating loss carryforwards tax benefits |  | 6,000,000 | 6,000,000 |  |  |
State and Local |  |  |  |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |  |  |
Operating loss carryforwards tax benefits |  | 49,000,000 | 49,000,000 |  |  |
Non-U.S. |  |  |  |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |  |  |
Operating loss carryforwards tax benefits |  | 31,000,000 | 31,000,000 |  |  |
Minimum |  |  |  |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |  |  |
Decrease in unrecognized tax benefits is reasonably possible |  | 0 | 0 |  |  |
Maximum |  |  |  |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |  |  |
Decrease in unrecognized tax benefits is reasonably possible |  | $ 6,000,000 | $ 6,000,000 |  |  |
Forecast |  |  |  |  |  |
Tax Credit Carryforward [Line Items] |  |  |  |  |  |
U.S. Federal statutory rate | 21.00% |  |  |  |  |

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