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HISTORICAL LOSS AND LAE DEVELOPMENT - Cumulative Paid Loss and Loss Adjustment Expenses, Net of Reinsurance (Details)
Dec. 31, 2017
Property segment  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Average annual percentage payout of incurred losses by age, Year 1 51.50%
Average annual percentage payout of incurred losses by age, Year 2 31.60%
Average annual percentage payout of incurred losses by age, Year 3 7.40%
Average annual percentage payout of incurred losses by age, Year 4 3.60%
Average annual percentage payout of incurred losses by age, Year 5 3.10%
Average annual percentage payout of incurred losses by age, Year 6 0.90%
Average annual percentage payout of incurred losses by age, Year 7 0.70%
Average annual percentage payout of incurred losses by age, Year 8 0.60%
Average annual percentage payout of incurred losses by age, Year 9 0.20%
Average annual percentage payout of incurred losses by age, Year 10 0.10%
Surety segment  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Average annual percentage payout of incurred losses by age, Year 1 23.80%
Average annual percentage payout of incurred losses by age, Year 2 42.00%
Average annual percentage payout of incurred losses by age, Year 3 13.60%
Average annual percentage payout of incurred losses by age, Year 4 4.60%
Average annual percentage payout of incurred losses by age, Year 5 4.00%
Average annual percentage payout of incurred losses by age, Year 6 2.80%
Average annual percentage payout of incurred losses by age, Year 7 0.80%
Average annual percentage payout of incurred losses by age, Year 8 (2.90%)
Average annual percentage payout of incurred losses by age, Year 9 19.30%
Average annual percentage payout of incurred losses by age, Year 10 0.40%
Primary Occurrence | Casualty segment  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Average annual percentage payout of incurred losses by age, Year 1 8.80%
Average annual percentage payout of incurred losses by age, Year 2 11.90%
Average annual percentage payout of incurred losses by age, Year 3 17.00%
Average annual percentage payout of incurred losses by age, Year 4 19.00%
Average annual percentage payout of incurred losses by age, Year 5 15.50%
Average annual percentage payout of incurred losses by age, Year 6 8.70%
Average annual percentage payout of incurred losses by age, Year 7 6.10%
Average annual percentage payout of incurred losses by age, Year 8 4.30%
Average annual percentage payout of incurred losses by age, Year 9 2.50%
Average annual percentage payout of incurred losses by age, Year 10 2.70%
Excess Occurrence | Casualty segment  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Average annual percentage payout of incurred losses by age, Year 1 4.10%
Average annual percentage payout of incurred losses by age, Year 2 16.40%
Average annual percentage payout of incurred losses by age, Year 3 21.90%
Average annual percentage payout of incurred losses by age, Year 4 19.20%
Average annual percentage payout of incurred losses by age, Year 5 8.80%
Average annual percentage payout of incurred losses by age, Year 6 7.80%
Average annual percentage payout of incurred losses by age, Year 7 6.20%
Average annual percentage payout of incurred losses by age, Year 8 2.60%
Average annual percentage payout of incurred losses by age, Year 9 0.10%
Average annual percentage payout of incurred losses by age, Year 10 0.70%
Claims Made | Casualty segment  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Average annual percentage payout of incurred losses by age, Year 1 2.80%
Average annual percentage payout of incurred losses by age, Year 2 14.80%
Average annual percentage payout of incurred losses by age, Year 3 18.40%
Average annual percentage payout of incurred losses by age, Year 4 14.70%
Average annual percentage payout of incurred losses by age, Year 5 8.90%
Average annual percentage payout of incurred losses by age, Year 6 14.70%
Average annual percentage payout of incurred losses by age, Year 7 26.70%
Average annual percentage payout of incurred losses by age, Year 8 6.20%
Average annual percentage payout of incurred losses by age, Year 9 0.80%
Average annual percentage payout of incurred losses by age, Year 10 0.00%
Transportation | Casualty segment  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Average annual percentage payout of incurred losses by age, Year 1 19.30%
Average annual percentage payout of incurred losses by age, Year 2 18.70%
Average annual percentage payout of incurred losses by age, Year 3 19.40%
Average annual percentage payout of incurred losses by age, Year 4 20.30%
Average annual percentage payout of incurred losses by age, Year 5 10.80%
Average annual percentage payout of incurred losses by age, Year 6 5.40%
Average annual percentage payout of incurred losses by age, Year 7 1.30%
Average annual percentage payout of incurred losses by age, Year 8 0.40%
Average annual percentage payout of incurred losses by age, Year 9 0.00%
Average annual percentage payout of incurred losses by age, Year 10 0.10%