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Summary of Significant Accounting Policies (Narratives) (Details) | 3 Months Ended | 12 Months Ended |  |
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Dec. 31, 2017 USD ($) shares | Sep. 30, 2017 USD ($) | Jun. 30, 2017 USD ($) | Mar. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) shares | Sep. 30, 2016 USD ($) | Jun. 30, 2016 USD ($) | Mar. 31, 2016 USD ($) | Dec. 31, 2017 USD ($) segment shares | Dec. 31, 2016 USD ($) shares | Dec. 31, 2015 USD ($) shares | Mar. 31, 2018 USD ($) | Jan. 01, 2018 USD ($)
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Number of business segments | segment |  |  |  |  |  |  |  |  | 2 |  |  |  |
Uncollected premiums and claims, net of related payables | $ 6,800,000,000 |  |  |  | $ 7,000,000,000 |  |  |  | $ 6,800,000,000 | $ 7,000,000,000 |  |  |
Restricted cash and cash equivalents | 187,000,000 |  |  |  |  |  |  |  | 187,000,000 |  |  |  |
Investment income (less than) |  |  |  |  |  |  |  |  | 15,000,000 | 1,000,000 | $ 38,000,000 |  |
Indefinite-lived intangible assets | 0 |  |  |  | 0 |  |  |  | 0 | 0 |  |  |
Capitalized computer software costs | 488,000,000 |  |  |  | 482,000,000 |  |  |  | 488,000,000 | 482,000,000 |  |  |
Capitalized computer software costs accumulated amortization | $ 1,300,000,000 |  |  |  | $ 1,100,000,000 |  |  |  | $ 1,300,000,000 | $ 1,100,000,000 |  |  |
Stock options outstanding (in shares) | shares | 10,200,702 |  |  |  | 13,242,529 |  |  |  | 10,200,702 | 13,242,529 | 14,800,000 |  |
Retained earnings | $ 13,140,000,000 |  |  |  | $ 12,388,000,000 |  |  |  | $ 13,140,000,000 | $ 12,388,000,000 |  |  |
Operating Income (Loss) | 686,000,000 | $ 535,000,000 | $ 701,000,000 | $ 749,000,000 | $ 580,000,000 | $ 517,000,000 | $ 661,000,000 | $ 673,000,000 | 2,655,000,000 | 2,431,000,000 | $ 2,184,000,000 |  |
Effective income tax rate reconciliation, share-based compensation, excess tax benefit, amount |  |  |  |  |  |  |  |  | 79,000,000 |  |  |  |
Adjustments to additional paid in capital, income tax benefit from share-based compensation |  |  |  |  |  |  |  |  |  | 44,000,000 | 53,000,000 |  |
Mercer Consulting Group |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Assets in trusts or funds for management or trustee fee | 227,000,000,000 |  |  |  |  |  |  |  | $ 227,000,000,000 |  |  |  |
Minimum | Software Development |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Property, plant and equipment, useful life |  |  |  |  |  |  |  |  | 3 years |  |  |  |
Maximum | Software Development |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Property, plant and equipment, useful life |  |  |  |  |  |  |  |  | 10 years |  |  |  |
Risk and Insurance Services Segment |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest income, fiduciary assets |  |  |  |  |  |  |  |  | $ 39,000,000 | 26,000,000 | 21,000,000 |  |
Consulting Segment |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Interest income, fiduciary assets |  |  |  |  |  |  |  |  | $ 4,000,000 | $ 3,000,000 | $ 4,000,000 |  |
Customer Lists | Minimum |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible asset useful life |  |  |  |  |  |  |  |  | 10 years |  |  |  |
Customer Lists | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Intangible asset useful life |  |  |  |  |  |  |  |  | 15 years |  |  |  |
Previously Reported |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Retained earnings | 325,000,000 |  |  |  |  |  |  |  | $ 325,000,000 |  |  |  |
Forecast |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative effect of new accounting principle in period of adoption |  |  |  |  |  |  |  |  |  |  |  | $ 15,000,000 |
Retained earnings |  |  |  |  |  |  |  |  |  |  |  |  | $ 425,000,000
Retained Earnings |  |  |  |  |  |  |  |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |
Cumulative effect of new accounting principle in period of adoption | $ 14,000,000 |  |  |  |  |  |  |  | $ 14,000,000 |  |  |  |

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