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Reserve for Losses and Loss Expenses Reserve for Losses and Loss Expenses (Historical Claims Duration) (Details)	Dec. 31, 2016
General Liability [Member] | Standard Commercial Lines [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	6.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	12.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	15.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	16.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	15.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	10.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	5.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	4.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	2.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	2.20%
General Liability [Member] | E&S Lines [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	3.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	12.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	17.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	19.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	16.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	9.50%
Workers Compensation [Member] | Standard Commercial Lines [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	19.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	23.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	14.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	8.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	5.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	3.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	3.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	2.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	2.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	1.20%
Commercial Automobile [Member] | Standard Commercial Lines [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	38.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	17.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	14.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	12.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	9.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	3.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	1.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	1.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.20%
Business Owners' Policies [Member] | Standard Commercial Lines [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	46.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	20.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	8.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	8.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	6.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	4.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	1.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	1.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.80%
Commercial Property [Member] | Standard Commercial Lines [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	70.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	26.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	2.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	0.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	0.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	0.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	0.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.10%
Personal Automobile [Member] | Standard Personal Lines [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	54.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	18.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	11.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	7.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	4.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	1.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	0.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.00%
Homeowners [Member] | Standard Personal Lines [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	69.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	21.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	3.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	1.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	1.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	0.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	0.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.10%

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+ References

No definition available.

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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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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
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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
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-Section 50
-Paragraph 4G
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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
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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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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
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-Section 50
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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
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-Topic 944
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-Section 50
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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
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-Topic 944
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-Section 50
-Paragraph 4G
-URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671329-158438

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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
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-Topic 944
-SubTopic 40
-Section 50
-Paragraph 4G
-URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671329-158438

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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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-Topic 944
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-URI http://asc.fasb.org/extlink&oid=65670685&loc=SL65671329-158438

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