Loss and Loss Expense Reserves, Average Historical Claims Duration (Details)	12 Months Ended
Dec. 31, 2017 Quarter yr
Minimum [Member]
Reserve Methodologies for Incurred But Not Reported Losses [Abstract]
Number of running accident quarters used \| Quarter	12
Number of accident years included in loss development triangles \| yr	15
Workers' Compensation [Member]
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance [Abstract]
Year One	17.60%
Year Two	26.50%
Year Three	15.50%
Year Four	9.70%
Year Five	5.60%
Year Six	4.10%
Year Seven	3.00%
Year Eight	1.90%
Year Nine	1.70%
Year Ten	2.80%
Professional Liability Assumed [Member]
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance [Abstract]
Year One	1.30%
Year Two	3.70%
Year Three	16.00%
Year Four	20.40%
Year Five	19.20%
Year Six	17.80%
Year Seven	10.20%
Year Eight	1.70%
Commercial Liability [Member]
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance [Abstract]
Year One	9.10%
Year Two	31.10%
Year Three	25.30%
Year Four	22.30%
Year Five	7.00%
Year Six	2.90%
Year Seven	1.80%
Year Eight	0.10%
Year Nine	0.30%
Year Ten	0.10%

No definition available.

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- Definition

The number of running accident quarters used to project the reserve necessary for incurred but not reported losses for short-duration insurance contracts.

+ References

No definition available.

+ Details

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- Definition

The number of accident years is included in the loss development triangles used to calculate link ratios and the selected loss development factors used to determine the reserves for incurred but not reported losses.

+ References

No definition available.

+ Details

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- References

No definition available.

+ Details

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- 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

+ Details

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- 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

+ Details

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- 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

+ Details

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- 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

+ Details

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- 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

+ Details

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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

+ Details

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- 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

+ Details

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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

+ Details

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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

+ Details

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- 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

+ Details

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