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Summary of Significant Accounting Policies (Narratives) (Details) | 9 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | ---
Sep. 30, 2018 USD ($) | Dec. 31, 2018 USD ($) segment | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | 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 |  | $ 7,300,000,000 | $ 6,800,000,000 |  |  |
Restricted cash and cash equivalents |  | 186,000,000 |  |  |  |
Investment loss |  | (12,000,000) |  |  |  |
Investment income (less than) |  |  | 15,000,000 | $ 1,000,000 |  |
Impairment charge | $ 83,000,000 |  |  |  |  |
Indefinite-lived intangible assets |  | 0 | 0 |  |  |
Capitalized computer software costs |  | 435,000,000 | 488,000,000 |  |  |
Capitalized computer software costs accumulated amortization |  | 1,300,000,000 | 1,300,000,000 |  |  |
Retained earnings |  | 14,347,000,000 | 13,140,000,000 |  |  | $ 13,504,000,000
Effective income tax rate reconciliation, share-based compensation, excess tax benefit, amount |  | 28,000,000 | 79,000,000 |  |  |
Adjustments to additional paid in capital, income tax benefit from share-based compensation |  |  |  | 44,000,000 |  |
Unrealized investment (loss) income |  | 0 | (7,000,000) | 21,000,000 |  |
Debt and Equity Securities, Realized Gain (Loss) |  | $ 17,000,000 |  |  |  |
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 |  | $ 65,000,000 | 39,000,000 | 26,000,000 |  |
Consulting Segment |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Interest income, fiduciary assets |  | $ 3,000,000 | 4,000,000 | $ 3,000,000 |  |
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 |  |  |  |
Retained Earnings |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Cumulative effect of amended accounting standard |  |  | $ 14,000,000 |  |  | 14,000,000
Accounting Standards Update 2014-09 |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Retained earnings |  |  |  |  | $ 364,000,000 | 364,000,000
Accounting Standards Update 2016-16 |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Retained earnings |  |  |  |  |  | $ (14,000,000)
Other Liabilities | Minimum |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Operating lease, liability |  | $ 200,000,000 |  |  |  |
Real Estate | Minimum |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Operating lease, liability |  | 1,800,000,000 |  |  |  |
Right-of-use asset obtained in exchange for operating lease liability |  | 1,600,000,000 |  |  |  |
Benefitfocus |  |  |  |  |  |
Schedule of Equity Method Investments [Line Items] |  |  |  |  |  |
Unrealized investment (loss) income |  | $ 54,000,000 |  |  |  |

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