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Reserve for Loss and Loss Expense (Historical Claims Duration) (Details) $ in Thousands	Dec. 31, 2021 USD ($)
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Net outstanding liabilities	$ 12,898,946
Standard Commercial Lines [Member] | General Liability [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	6.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	11.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	15.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	17.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	14.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	9.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	6.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	5.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	1.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	1.60%
Standard Commercial Lines [Member] | Workers Compensation [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	21.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	25.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	13.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	8.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	4.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	2.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	2.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	3.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	1.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.80%
Standard Commercial Lines [Member] | Commercial Automobile [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	37.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	16.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	14.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	13.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	10.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	4.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	1.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	1.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.10%
Standard Commercial Lines [Member] | Business Owners' Policies [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	49.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	20.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	8.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	8.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	5.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	3.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	1.20%
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%
Standard Commercial Lines [Member] | Commercial Property [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	69.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	25.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	3.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	1.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	0.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.00%
Standard Personal Lines | Personal Automobile [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	59.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	18.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	10.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	6.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	3.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	1.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	0.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.10%
Standard Personal Lines | Homeowners [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	71.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	21.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	3.40%
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	0.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	0.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.00%
E&S Lines [Member] | General Liability [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	3.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	11.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	16.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	19.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	14.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	7.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	5.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	3.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	3.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	2.00%

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.

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Undiscounted amount, after reinsurance, of incurred claims and allocated claim adjustment expense used in claims development for short-duration insurance contracts. Excludes unallocated claim adjustment expense.

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