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Reserves For Losses And Loss Expenses (Supplementary Information Regarding Average Historical Claims Duration) (Details)
Dec. 31, 2020
Insurance | Other liability  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 1 7.50%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 2 14.40%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 3 17.70%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 4 16.60%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 5 13.40%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 6 8.70%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 7 5.60%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 8 3.50%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 9 2.60%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 10 1.90%
Insurance | Workers' compensation  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 1 23.20%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 2 29.40%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 3 15.90%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 4 9.30%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 5 5.90%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 6 3.50%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 7 2.40%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 8 1.60%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 9 1.50%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 10 1.10%
Insurance | Professional liability  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 1 8.40%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 2 20.70%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 3 19.50%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 4 16.40%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 5 10.10%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 6 8.30%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 7 4.00%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 8 2.00%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 9 2.30%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 10 0.80%
Insurance | Commercial automobile  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 1 37.70%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 2 21.50%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 3 15.30%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 4 10.90%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 5 6.60%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 6 2.90%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 7 1.40%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 8 0.40%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 9 0.20%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 10 0.30%
Insurance | Short-tail lines  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 1 57.00%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 2 31.70%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 3 6.00%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 4 1.60%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 5 0.40%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 6 0.50%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 7 0.40%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 8 0.30%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 9 0.60%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 10 0.20%
Reinsurance & Monoline Excess | Casualty  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 1 6.50%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 2 11.70%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 3 14.10%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 4 14.00%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 5 11.50%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 6 8.90%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 7 6.50%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 8 4.70%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 9 4.90%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 10 3.90%
Reinsurance & Monoline Excess | Monoline excess  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 1 4.90%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 2 4.50%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 3 2.80%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 4 3.10%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 5 2.40%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 6 3.50%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 7 4.30%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 8 2.90%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 9 3.50%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 10 1.70%
Reinsurance & Monoline Excess | Property  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 1 33.30%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 2 33.20%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 3 15.40%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 4 6.00%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 5 3.90%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 6 2.20%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 7 1.60%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 8 1.20%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 9 0.80%
Average annual percentage payout of incurred claims by age, net of reinsurance, Year 10 0.60%