| **Claim and Claim Adjustment Expense Reserves (Historical Claims Duration) (Details)** | Dec. 31, 2019 |
|----------------------------------------------------------------------------------------|---------------|
| Medical Professional Liability | Specialty                                             |          |
| **Short-duration Insurance Contracts, Historical Claims Duration [Line Items]**        |          |
| Year 1                                                                                 | 3.80%         |
| Year 2                                                                                 | 20.00%        |
| Year 3                                                                                 | 24.30%        |
| Year 4                                                                                 | 19.40%        |
| Year 5                                                                                 | 12.40%        |
| Year 6                                                                                 | 7.80%         |
| Year 7                                                                                 | 5.10%         |
| Year 8                                                                                 | 3.10%         |
| Year 9                                                                                 | 1.10%         |
| Year 10                                                                                | 0.80%         |
| Total                                                                                  | 97.80%        |
| Other Professional Liability and Management Liability | Specialty                      |          |
| **Short-duration Insurance Contracts, Historical Claims Duration [Line Items]**        |          |
| Year 1                                                                                 | 6.60%         |
| Year 2                                                                                 | 22.60%        |
| Year 3                                                                                 | 21.70%        |
| Year 4                                                                                 | 16.90%        |
| Year 5                                                                                 | 10.50%        |
| Year 6                                                                                 | 6.00%         |
| Year 7                                                                                 | 4.90%         |
| Year 8                                                                                 | 3.30%         |
| Year 9                                                                                 | 3.80%         |
| Year 10                                                                                | 0.70%         |
| Total                                                                                  | 97.00%        |
| Surety | Specialty                                                                     |          |
| **Short-duration Insurance Contracts, Historical Claims Duration [Line Items]**        |          |
| Year 1                                                                                 | 20.00%        |
| Year 2                                                                                 | 44.50%        |
| Year 3                                                                                 | 21.00%        |
| Year 4                                                                                 | 4.40%         |
| Year 5                                                                                 | 2.20%         |
| Year 6                                                                                 | 0.90%         |
| Year 7                                                                                 | (2.50%)       |
| Year 8                                                                                 | (1.10%)       |
| Year 9                                                                                 | 0.00%         |
| Year 10                                                                                | 1.60%         |
| Total                                                                                  | 91.00%        |
| Commercial Auto | Commercial                                                           |          |
| **Short-duration Insurance Contracts, Historical Claims Duration [Line Items]**        |          |
| Year 1                                                                                 | 28.60%        |
| Year 2                                                                                 | 24.00%        |
| Year 3                                                                                 | 18.60%        |
| Year 4                                                                                 | 14.30%        |
| Year 5                                                                                 | 9.40%         |
| Year 6                                                                                 | 3.20%         |
| Year 7                                                                                 | 1.50%         |
| Year 8                                                                                 | 0.40%         |
| Year 9                                                                                 | 0.00%         |
| Year 10                                                                                | 0.00%         |
| Total                                                                                  | 100.00%       |
| General Liability | Commercial                                                         |          |
| **Short-duration Insurance Contracts, Historical Claims Duration [Line Items]**        |          |
| Year 1                                                                                 | 4.30%         |
| Year 2                                                                                 | 15.80%        |
| Year 3                                                                                 | 19.00%        |
| Year 4                                                                                 | 20.10%        |
| Year 5                                                                                 | 15.70%        |
| Year 6                                                                                 | 8.70%         |
| Year 7                                                                                 | 6.00%         |
| Year 8                                                                                 | 2.50%         |
| Year 9                                                                                 | 1.50%         |
| Year 10                                                                                | 1.60%         |
| Total                                                                                  | 95.20%        |
| Workers' Compensation | Commercial                                                     |          |
| **Short-duration Insurance Contracts, Historical Claims Duration [Line Items]**        |          |
| Year 1                                                                                 | 14.10%        |
| Year 2                                                                                 | 21.40%        |
| Year 3                                                                                 | 13.90%        |
| Year 4                                                                                 | 10.30%        |
| Year 5                                                                                 | 7.00%         |
| Year 6                                                                                 | 3.70%         |
| Year 7                                                                                 | 2.90%         |
| Year 8                                                                                 | 2.70%         |
| Year 9                                                                                 | 1.40%         |
| Year 10                                                                                | 0.80%         |
| Total                                                                                  | 78.20%        |
| Excluding Hardy | International                                                        |          |
| **Short-duration Insurance Contracts, Historical Claims Duration [Line Items]**        |          |
| Year 1                                                                                 | 20.80%        |
| Year 2                                                                                 | 25.70%        |
| Year 3                                                                                 | 11.00%        |
| Year 4                                                                                 | 7.30%         |
| Year 5                                                                                 | 6.80%         |
| Year 6                                                                                 | 5.30%         |
| Year 7                                                                                 | 5.00%         |
| Year 8                                                                                 | 1.70%         |
| Year 9                                                                                 | 1.20%         |
| Year 10                                                                                | 2.10%         |
| Total                                                                                  | 86.90%        |
| Hardy | International                                                                  |          |
| **Short-duration Insurance Contracts, Historical Claims Duration [Line Items]**        |          |
| Year 1                                                                                 | 23.30%        |
| Year 2                                                                                 | 39.60%        |
| Year 3                                                                                 | 8.70%         |
| Year 4                                                                                 | 5.60%         |
| Year 5                                                                                 | 4.50%         |
| Year 6                                                                                 | 2.50%         |
| Year 7                                                                                 | 2.80%         |
| Total                                                                                  | 87.00%        |

| X                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Short-Duration Insurance Contracts, Historical Claims Duration, Net
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
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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
Percentage of average annual payout, after reinsurance, in the eighth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of average annual payout, after reinsurance, in the fifth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of average annual payout, after reinsurance, in the fourth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of average annual payout, after reinsurance, in the ninth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of average annual payout, after reinsurance, in the first year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of average annual payout, after reinsurance, in the seventh year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of average annual payout, after reinsurance, in the sixth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                          |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of average annual payout, after reinsurance, in the tenth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
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| - Definition
Percentage of average annual payout, after reinsurance, in the third year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                           |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Percentage of average annual payout, after reinsurance, in the second year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.
+ References
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