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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 |  |  |

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