| **Claim and Claim Adjustment Expense Reserves Short-Duration Contracts - Historical Claims Duration (Details)** | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009 | Dec. 31, 2008 | Dec. 31, 2007 |
|-----------------------------------------------------------------------------------------------------------------|---------------|---------------|---------------|---------------|---------------|---------------|---------------|---------------|---------------|---------------|
| Specialty - Medical Professional Liability [Member]                                                             |          |          |          |          |          |          |          |          |          |          |
| **Short-duration Insurance Contracts, Historical Claims Duration [Line Items]**                                 |          |          |          |          |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year One                                        |          |          |          |          |          |          |          |          |          | 3.10%         |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Two                                        |          |          |          |          |          |          |          |          | 18.20%        |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Three                                      |          |          |          |          |          |          |          | 22.30%        |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Four                                       |          |          |          |          |          |          | 19.60%        |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Five                                       |          |          |          |          |          | 12.70%        |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Six                                        |          |          |          |          | 8.00%         |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Seven                                      |          |          |          | 5.00%         |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Eight                                      |          |          | 4.80%         |          |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Nine                                       |          | 3.70%         |          |          |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Ten                                        | 0.30%         |          |          |          |          |          |          |          |          |          |
| Short-Duration Insurance Contracts, Historical Claims Duration, Net                                             | 97.70%        |          |          |          |          |          |          |          |          |          |
| Specialty - Other Professional and Management Liability [Member]                                                |          |          |          |          |          |          |          |          |          |          |
| **Short-duration Insurance Contracts, Historical Claims Duration [Line Items]**                                 |          |          |          |          |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year One                                        |          |          |          |          |          |          |          |          |          | 5.70%         |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Two                                        |          |          |          |          |          |          |          |          | 20.60%        |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Three                                      |          |          |          |          |          |          |          | 21.00%        |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Four                                       |          |          |          |          |          |          | 17.00%        |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Five                                       |          |          |          |          |          | 10.00%        |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Six                                        |          |          |          |          | 6.30%         |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Seven                                      |          |          |          | 5.60%         |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Eight                                      |          |          | 3.80%         |          |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Nine                                       |          | 1.90%         |          |          |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Ten                                        | 3.20%         |          |          |          |          |          |          |          |          |          |
| Short-Duration Insurance Contracts, Historical Claims Duration, Net                                             | 95.10%        |          |          |          |          |          |          |          |          |          |
| Specialty - Surety [Member]                                                                                     |          |          |          |          |          |          |          |          |          |          |
| **Short-duration Insurance Contracts, Historical Claims Duration [Line Items]**                                 |          |          |          |          |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year One                                        |          |          |          |          |          |          |          |          |          | 23.40%        |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Two                                        |          |          |          |          |          |          |          |          | 32.80%        |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Three                                      |          |          |          |          |          |          |          | 20.20%        |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Four                                       |          |          |          |          |          |          | 8.90%         |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Five                                       |          |          |          |          |          | 3.70%         |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Six                                        |          |          |          |          | 1.50%         |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Seven                                      |          |          |          | (0.70%)       |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Eight                                      |          |          | 2.00%         |          |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Nine                                       |          | 1.00%         |          |          |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Ten                                        | 2.00%         |          |          |          |          |          |          |          |          |          |
| Short-Duration Insurance Contracts, Historical Claims Duration, Net                                             | 94.80%        |          |          |          |          |          |          |          |          |          |
| Commercial - Auto [Member]                                                                                      |          |          |          |          |          |          |          |          |          |          |
| **Short-duration Insurance Contracts, Historical Claims Duration [Line Items]**                                 |          |          |          |          |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year One                                        |          |          |          |          |          |          |          |          |          | 27.20%        |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Two                                        |          |          |          |          |          |          |          |          | 23.10%        |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Three                                      |          |          |          |          |          |          |          | 18.30%        |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Four                                       |          |          |          |          |          |          | 13.90%        |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Five                                       |          |          |          |          |          | 9.10%         |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Six                                        |          |          |          |          | 3.90%         |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Seven                                      |          |          |          | 1.30%         |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Eight                                      |          |          | 0.20%         |          |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Nine                                       |          | 0.30%         |          |          |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Ten                                        | 0.30%         |          |          |          |          |          |          |          |          |          |
| Short-Duration Insurance Contracts, Historical Claims Duration, Net                                             | 97.60%        |          |          |          |          |          |          |          |          |          |
| Commercial - General Liability [Member]                                                                         |          |          |          |          |          |          |          |          |          |          |
| **Short-duration Insurance Contracts, Historical Claims Duration [Line Items]**                                 |          |          |          |          |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year One                                        |          |          |          |          |          |          |          |          |          | 5.00%         |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Two                                        |          |          |          |          |          |          |          |          | 16.30%        |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Three                                      |          |          |          |          |          |          |          | 20.50%        |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Four                                       |          |          |          |          |          |          | 20.10%        |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Five                                       |          |          |          |          |          | 15.30%        |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Six                                        |          |          |          |          | 8.00%         |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Seven                                      |          |          |          | 4.20%         |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Eight                                      |          |          | 2.70%         |          |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Nine                                       |          | 1.10%         |          |          |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Ten                                        | 1.30%         |          |          |          |          |          |          |          |          |          |
| Short-Duration Insurance Contracts, Historical Claims Duration, Net                                             | 94.50%        |          |          |          |          |          |          |          |          |          |
| Commercial - Workers Compensation [Member]                                                                      |          |          |          |          |          |          |          |          |          |          |
| **Short-duration Insurance Contracts, Historical Claims Duration [Line Items]**                                 |          |          |          |          |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year One                                        |          |          |          |          |          |          |          |          |          | 13.50%        |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Two                                        |          |          |          |          |          |          |          |          | 21.40%        |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Three                                      |          |          |          |          |          |          |          | 14.60%        |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Four                                       |          |          |          |          |          |          | 10.50%        |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Five                                       |          |          |          |          |          | 7.50%         |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Six                                        |          |          |          |          | 5.70%         |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Seven                                      |          |          |          | 4.60%         |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Eight                                      |          |          | 3.00%         |          |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Nine                                       |          | 2.10%         |          |          |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Ten                                        | 0.30%         |          |          |          |          |          |          |          |          |          |
| Short-Duration Insurance Contracts, Historical Claims Duration, Net                                             | 83.20%        |          |          |          |          |          |          |          |          |          |
| International Excluding Hardy [Member]                                                                          |          |          |          |          |          |          |          |          |          |          |
| **Short-duration Insurance Contracts, Historical Claims Duration [Line Items]**                                 |          |          |          |          |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year One                                        |          |          |          |          |          |          |          |          |          | 19.50%        |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Two                                        |          |          |          |          |          |          |          |          | 27.40%        |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Three                                      |          |          |          |          |          |          |          | 12.10%        |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Four                                       |          |          |          |          |          |          | 6.80%         |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Five                                       |          |          |          |          |          | 5.90%         |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Six                                        |          |          |          |          | 4.50%         |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Seven                                      |          |          |          | 4.80%         |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Eight                                      |          |          | 3.70%         |          |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Nine                                       |          | 5.00%         |          |          |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Ten                                        | 1.30%         |          |          |          |          |          |          |          |          |          |
| Short-Duration Insurance Contracts, Historical Claims Duration, Net                                             | 91.00%        |          |          |          |          |          |          |          |          |          |
| Hardy [Member]                                                                                                  |          |          |          |          |          |          |          |          |          |          |
| **Short-duration Insurance Contracts, Historical Claims Duration [Line Items]**                                 |          |          |          |          |          |          |          |          |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year One                                        |          |          |          |          |          |          |          |          |          | 25.70%        |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Two                                        |          |          |          |          |          |          |          |          | 40.90%        |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Three                                      |          |          |          |          |          |          |          | 12.10%        |          |          |
| Short-duration Insurance Contracts, Historical Claims Duration, Year Four                                       |          |          |          |          |          |          | 4.40%         |          |          |          |
| Short-Duration Insurance Contracts, Historical Claims Duration, Net                                             | 83.10%        |          |          |          |          |          |          |          |          |          |

| X                                                                                                                                                                                                                                                                                                           |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Short-Duration Insurance Contracts, Historical Claims Duration, Net
+ References
No definition available.
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| - 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
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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4G -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671329-158438
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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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4G -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671329-158438
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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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4G -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671329-158438
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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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4G -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671329-158438
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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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4G -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671329-158438
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
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| - 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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4G -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671329-158438
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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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4G -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671329-158438
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                            |
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| - 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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4G -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671329-158438
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - 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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4G -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671329-158438
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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
Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 4G -URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671329-158438
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