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Reserve for Claims and Claim Expenses (Historical Claims Duration) (Details)
Dec. 31, 2019
Property  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Average annual percentage payout, year one 29.90%
Average annual percentage payout, year two 17.00%
Average annual percentage payout, year three 17.80%
Average annual percentage payout, year four 8.70%
Average annual percentage payout, year five 5.40%
Average annual percentage payout, year six 2.70%
Average annual percentage payout, year seven 4.90%
Average annual percentage payout, year eight 1.00%
Average annual percentage payout, year nine 1.20%
Average annual percentage payout, year ten 1.50%
Casualty and Specialty  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Average annual percentage payout, year one 14.30%
Average annual percentage payout, year two 18.40%
Average annual percentage payout, year three 13.00%
Average annual percentage payout, year four 10.30%
Average annual percentage payout, year five 8.80%
Average annual percentage payout, year six 8.20%
Average annual percentage payout, year seven 4.60%
Average annual percentage payout, year eight 4.70%
Average annual percentage payout, year nine 1.70%
Average annual percentage payout, year ten 1.40%