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Reserve for Loss and Loss Expense (Historical Claims Duration) (Details) $ in Thousands	Dec. 31, 2022 USD ($)
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Net outstanding liabilities	$ 13,946,270
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	6.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	11.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	15.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	17.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	15.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	9.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	7.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	4.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	3.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	2.10%
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	22.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	25.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	13.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	8.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	4.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	3.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	1.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	1.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	3.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	2.20%
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	36.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	16.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	14.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	13.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	9.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	4.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	1.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	1.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.70%
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	49.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	21.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	8.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	8.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	6.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	3.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	1.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.20%
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	68.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	26.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	3.20%
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	59.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	18.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	8.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	6.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	3.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	1.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	0.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.20%
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	70.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	21.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	3.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	1.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	1.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	0.20%
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	0.00%
E&S Lines [Member] | Casualty Insurance Product Line [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	11.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	18.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	21.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	15.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	7.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	6.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	5.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	2.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	3.50%

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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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