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Summary of Significant Accounting Policies Losses, Claims and Settlement Expenses - Consolidated - Historical Average Annual Payout of Incurred Claims (Details)
Dec. 31, 2020
Workers' compensation [Member]  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Year 1 15.50%
Year 2 22.60%
Year 3 13.70%
Year 4 8.70%
Year 5 5.60%
Year 6 3.80%
Year 7 3.20%
Year 8 1.90%
Year 9 1.40%
Year 10 1.20%
General liability [Member]  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Year 1 4.90%
Year 2 12.50%
Year 3 16.20%
Year 4 17.90%
Year 5 15.70%
Year 6 8.40%
Year 7 6.10%
Year 8 3.70%
Year 9 0.60%
Year 10 0.00%
Commercial automobile [Member]  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Year 1 37.00%
Year 2 23.40%
Year 3 16.10%
Year 4 10.50%
Year 5 5.20%
Year 6 2.30%
Year 7 0.20%
Year 8 (0.20%)
Year 9 0.20%
Year 10 0.10%