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Reserve for Loss and Loss Expense (Historical Claims Duration) (Details) $ in Thousands	Dec. 31, 2023 USD ($)
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Net outstanding liabilities	$ 15,332,846
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	6.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	11.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	14.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	16.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	14.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	10.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	8.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	4.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	2.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	3.40%
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	22.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	26.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	13.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	9.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	4.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	2.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	2.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	3.20%
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	17.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	14.20%
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	3.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	2.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.60%
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	49.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	21.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	8.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	8.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	5.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	2.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	1.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.40%
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	68.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	26.30%
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	0.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	0.10%
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%
Standard Personal Lines | Personal Automobile [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	59.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	19.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	8.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	5.90%
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	0.50%
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	70.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	22.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	3.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	1.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	1.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	0.30%
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%
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	3.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	11.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	17.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	20.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	14.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	8.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	6.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	5.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	2.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.90%

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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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