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Reserves for Losses and Loss Adjustment Expenses - Schedule of Supplementary Unaudited Information About Annual Percentage Payout of Incurred Losses and ALAE, Net of Reinsurance (Detail)
Dec. 31, 2020
U.S. Operations | Excess and Surplus Lines | Liability  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Year 1 6.70%
Year 2 15.70%
Year 3 20.90%
Year 4 20.80%
Year 5 12.70%
Year 6 8.50%
Year 7 5.10%
Year 8 3.30%
Year 9 2.10%
Year 10 1.30%
U.S. Operations | Specialty Admitted | Liability  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Year 1 14.10%
Year 2 22.00%
Year 3 18.60%
Year 4 16.30%
Year 5 9.60%
Year 6 6.10%
Year 7 4.00%
Year 8 2.70%
Year 9 1.80%
Year 10 1.30%
U.S. Operations | Specialty Admitted | Professional  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Year 1 6.10%
Year 2 22.80%
Year 3 27.30%
Year 4 20.40%
Year 5 9.20%
Year 6 6.20%
Year 7 3.40%
Year 8 1.90%
Year 9 1.10%
Year 10 0.60%
U.S. Operations | Specialty Admitted | Specialty  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Year 1 53.70%
Year 2 11.60%
Year 3 28.70%
Year 4 3.60%
Year 5 1.40%
Year 6 0.60%
Year 7 0.20%
Year 8 0.10%
International Operations | Reinsurance Division | Property  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Year 1 24.90%
Year 2 34.00%
Year 3 20.70%
Year 4 14.50%
Year 5 2.50%
Year 6 1.20%
Year 7 0.70%
Year 8 0.40%
Year 9 0.30%
Year 10 0.20%
International Operations | Bermuda Insurance | Liability  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Year 2 20.80%
Year 3 15.60%
Year 4 14.80%
Year 5 12.20%
Year 6 9.30%
Year 7 6.90%
Year 8 5.10%
Year 9 3.70%
Year 10 2.80%
International Operations | Syndicate 1200 | Liability  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Year 1 5.80%
Year 2 12.10%
Year 3 13.40%
Year 4 15.00%
Year 5 16.00%
Year 6 11.70%
Year 7 6.90%
Year 8 5.20%
Year 9 3.70%
Year 10 2.70%
International Operations | Syndicate 1200 | Professional  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Year 1 6.10%
Year 2 13.70%
Year 3 18.50%
Year 4 17.60%
Year 5 12.40%
Year 6 8.70%
Year 7 5.90%
Year 8 4.50%
Year 9 3.20%
Year 10 2.30%
International Operations | Syndicate 1200 | Property  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Year 1 42.80%
Year 2 30.20%
Year 3 14.30%
Year 4 8.40%
Year 5 2.70%
Year 6 0.80%
Year 7 0.40%
Year 8 0.20%
Year 9 0.10%
International Operations | Syndicate 1200 | Specialty  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Year 1 37.40%
Year 2 31.00%
Year 3 16.70%
Year 4 9.20%
Year 5 3.00%
Year 6 1.50%
Year 7 0.60%
Year 8 0.30%
Year 9 0.10%