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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, 2019
U.S. Operations [Member] | Excess and Surplus Lines [Member] | Liability [Member]  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Year 1 7.60%
Year 2 16.80%
Year 3 20.50%
Year 4 18.60%
Year 5 12.60%
Year 6 8.30%
Year 7 5.30%
Year 8 3.40%
Year 9 2.20%
U.S. Operations [Member] | Specialty Admitted [Member] | Liability [Member]  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Year 1 14.10%
Year 2 21.30%
Year 3 18.40%
Year 4 16.40%
Year 5 9.30%
Year 6 6.30%
Year 7 4.20%
Year 8 2.80%
Year 9 1.90%
U.S. Operations [Member] | Specialty Admitted [Member] | Professional [Member]  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Year 1 7.70%
Year 2 25.20%
Year 3 26.30%
Year 4 16.90%
Year 5 10.30%
Year 6 5.80%
Year 7 3.20%
Year 8 1.80%
Year 9 1.10%
U.S. Operations [Member] | Specialty Admitted [Member] | Specialty [Member]  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Year 1 35.80%
Year 2 32.50%
Year 3 17.60%
Year 4 7.30%
Year 5 3.50%
Year 6 1.70%
Year 7 0.80%
Year 8 0.40%
Year 9 0.20%
International Operations [Member] | Reinsurance Division [Member] | Property [Member]  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Year 1 22.50%
Year 2 22.70%
Year 3 29.40%
Year 4 14.20%
Year 5 4.40%
Year 6 2.30%
Year 7 1.30%
Year 8 0.80%
Year 9 0.60%
International Operations [Member] | Bermuda Insurance [Member] | Liability [Member]  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Year 1 3.70%
Year 2 16.80%
Year 3 17.40%
Year 4 15.70%
Year 5 14.10%
Year 6 7.70%
Year 7 7.00%
Year 8 2.00%
Year 9 5.50%
International Operations [Member] | Europe [Member] | Professional [Member]  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Year 1 4.60%
Year 2 10.70%
Year 3 14.50%
Year 4 15.50%
Year 5 11.80%
Year 6 9.80%
Year 7 8.70%
International Operations [Member] | Syndicate 1200 [Member] | Liability [Member]  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Year 1 3.70%
Year 2 7.80%
Year 3 9.50%
Year 4 12.30%
Year 5 14.70%
Year 6 11.70%
Year 7 10.60%
Year 8 8.40%
Year 9 6.40%
International Operations [Member] | Syndicate 1200 [Member] | Professional [Member]  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Year 1 4.60%
Year 2 10.70%
Year 3 14.50%
Year 4 15.50%
Year 5 11.80%
Year 6 9.80%
Year 7 8.70%
Year 8 6.70%
Year 9 5.00%
International Operations [Member] | Syndicate 1200 [Member] | Property [Member]  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Year 1 41.50%
Year 2 29.60%
Year 3 15.70%
Year 4 8.40%
Year 5 2.40%
Year 6 1.20%
Year 7 0.60%
Year 8 0.30%
Year 9 0.20%
International Operations [Member] | Syndicate 1200 [Member] | Specialty [Member]  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Year 1 37.20%
Year 2 30.90%
Year 3 16.60%
Year 4 9.40%
Year 5 3.40%
Year 6 1.40%
Year 7 0.60%
Year 8 0.20%
Year 9 0.10%