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Principles of Consolidation And Other Matters (Details) - USD ($) | 3 Months Ended | 9 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2019 | Sep. 30, 2018 | Sep. 30, 2019 | Sep. 30, 2018 | Jan. 01, 2019 | Dec. 31, 2018
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items] |  |  |  |  |  |
Operating funds related to regulatory requirements or as collateral under captive insurance arrangements | $ 193,000,000 |  | $ 193,000,000 |  |  | $ 186,000,000
Equity method investments lag period |  |  | 3 months |  |  |
Gain (loss) on investment income, net | 7,000,000 | $ (52,000,000) | $ 20,000,000 | $ (24,000,000) |  |
Gains on equity securities | 4,000,000 | 25,000,000 | 10,000,000 | 43,000,000 |  |
Equity method income | 3,000,000 | $ 4,000,000 | 10,000,000 | $ 14,000,000 |  |
Operating lease, liability | 2,298,000,000 |  | 2,298,000,000 |  |  |
Operating lease, right-of-use asset | $ 1,957,000,000 |  | $ 1,957,000,000 |  |  | 0
Effective tax rate (as a percent) | 26.00% | 27.50% | 27.80% | 25.20% |  |
Unrecognized tax benefits | $ 76,000,000 |  | $ 76,000,000 |  |  | $ 78,000,000
Minimum |  |  |  |  |  |
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items] |  |  |  |  |  |
Reasonably possible decrease in unrecognized tax benefits | 0 |  | 0 |  |  |
Maximum |  |  |  |  |  |
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items] |  |  |  |  |  |
Reasonably possible decrease in unrecognized tax benefits | $ 7,000,000 |  | $ 7,000,000 |  |  |
Accounting Standards Update 2016-02 |  |  |  |  |  |
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items] |  |  |  |  |  |
Operating lease, liability |  |  |  |  | $ 1,900,000,000 |
Operating lease, right-of-use asset |  |  |  |  | 1,700,000,000 |
Restatement Adjustment | Accounting Standards Update 2016-02 |  |  |  |  |  |
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items] |  |  |  |  |  |
Other liabilities |  |  |  |  | $ 200,000,000 |
Alexander Forbes Group Holdings Limited |  |  |  |  |  |
Significant Change in Unrecognized Tax Benefits is Reasonably Possible [Line Items] |  |  |  |  |  |
Equity method investment other than temporary impairment charge |  | $ 81,000,000 |  | $ 81,000,000 |  |

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