| **Non-Insurance Revenues from Contracts with Customers (Performance Obligation) (Details) $ in Millions** | Mar. 31, 2019 USD ($) |
|-----------------------------------------------------------------------------------------------------------|-----------------------|
| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-04-01 |                  |
| **Revenue, Initial Application Period Cumulative Effect Transition [Line Items]**                         |                  |
| Remaining performance obligation, amount                                                                  | $ 754                 |
| Remaining performance obligation, expected timing of satisfaction, period                                 | 9 months              |
| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01 |                  |
| **Revenue, Initial Application Period Cumulative Effect Transition [Line Items]**                         |                  |
| Remaining performance obligation, amount                                                                  | $ 852                 |
| Remaining performance obligation, expected timing of satisfaction, period                                 | 1 year                |
| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01 |                  |
| **Revenue, Initial Application Period Cumulative Effect Transition [Line Items]**                         |                  |
| Remaining performance obligation, amount                                                                  | $ 673                 |
| Remaining performance obligation, expected timing of satisfaction, period                                 | 1 year                |
| Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2022-01-01 |                  |
| **Revenue, Initial Application Period Cumulative Effect Transition [Line Items]**                         |                  |
| Remaining performance obligation, amount                                                                  | $ 1,200               |
| Remaining performance obligation, expected timing of satisfaction, period                                 |                       |

| X                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of transaction price allocated to performance obligation that has not been recognized as revenue.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 13 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130556-203045
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Period in which remaining performance obligation is expected to be recognized as revenue, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.
+ References
Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 606 -SubTopic 10 -Section 50 -Paragraph 13 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130556-203045
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