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Summary of Significant Accounting Policies (Narrative) (Details) |  | 12 Months Ended |  |
--- | --- | --- | --- | ---
Mar. 31, 2019 USD ($) country segment | Dec. 31, 2020 USD ($) segment | Dec. 31, 2019 USD ($) | Dec. 31, 2018 USD ($) | Mar. 31, 2020 country | Jan. 01, 2019 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Number of business segments (segment) | segment |  | 2 |  |  |  |
Restricted cash |  | $ 270,000,000 |  |  |  |
Investment (loss) income |  | (22,000,000) | $ 22,000,000 | $ (12,000,000) |  |
Gains on equity securities |  | (27,000,000) | 10,000,000 | 54,000,000 |  |
Gain related to investments in private equity funds and other investments |  |  | 12,000,000 | 17,000,000 |  |
Indefinite lived identified intangible assets |  | 0 | 0 |  |  |
Capitalized computer software costs |  | 481,000,000 | 496,000,000 |  |  |
Accumulated amortization |  | 1,600,000,000 | 1,400,000,000 |  |  |
Liability for errors and omission, current and noncurrent |  | 639,000,000 | 484,000,000 |  |  |
Net uncollected premiums and claims and related payables |  | 11,200,000,000 | 8,900,000,000 |  |  |
Lease liability |  | 2,266,000,000 |  |  |  |
ROU asset | $ 1,700,000,000 | 1,894,000,000 | 1,921,000,000 |  |  |
Accounts Payable and Accrued Liabilities |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Liabilities for errors and omissions, current |  | 271,000,000 | 149,000,000 |  |  |
Cumulative Effect, Period of Adoption, Adjustment |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Lease liability |  |  |  |  |  | $ 1,900,000,000
ROU asset |  |  |  |  |  | 1,700,000,000
Other liabilities |  |  |  |  |  | $ (200,000,000)
Alexander Forbes |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Investment (loss) income |  | $ (23,000,000) |  |  |  |
Impairment charge |  |  |  | 83,000,000 |  |
Minimum | Customer relationships |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Intangible assets, useful life |  | 10 years |  |  |  |
Minimum | Furniture and equipment |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Property, plant and equipment, useful life |  | 3 years |  |  |  |
Minimum | Building |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Property, plant and equipment, useful life |  | 30 years |  |  |  |
Minimum | Software Development |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Property, plant and equipment, useful life |  | 3 years |  |  |  |
Maximum | Customer relationships |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Intangible assets, useful life |  | 15 years |  |  |  |
Maximum | Furniture and equipment |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Property, plant and equipment, useful life |  | 10 years |  |  |  |
Maximum | Building |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Property, plant and equipment, useful life |  | 40 years |  |  |  |
Maximum | Software Development |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Property, plant and equipment, useful life |  | 10 years |  |  |  |
JLT Transaction |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Number of business segments (segment) | segment | 3 |  |  |  |  |
Number of countries in which entity operates (country) | country | 41 |  |  |  | 41 |
Risk and insurance services |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Interest income, fiduciary assets |  | $ 46,000,000 | 105,000,000 | 65,000,000 |  |
Consulting Segment |  |  |  |  |  |
New Accounting Pronouncements or Change in Accounting Principle [Line Items] |  |  |  |  |  |
Interest income, fiduciary assets |  | $ 1,000,000 | $ 4,000,000 | $ 3,000,000 |  |

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