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Reserve for Claims and Claim Expenses (Historical Claims Duration) (Details)
Dec. 31, 2017
Property  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Average annual percentage payout, year one 27.80%
Average annual percentage payout, year two 23.00%
Average annual percentage payout, year three 21.30%
Average annual percentage payout, year four 10.10%
Average annual percentage payout, year five 5.20%
Average annual percentage payout, year six 3.10%
Average annual percentage payout, year seven 3.90%
Average annual percentage payout, year eight 0.90%
Average annual percentage payout, year nine 0.20%
Average annual percentage payout, year ten 0.10%
Casualty and Specialty  
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]  
Average annual percentage payout, year one 10.10%
Average annual percentage payout, year two 19.60%
Average annual percentage payout, year three 11.80%
Average annual percentage payout, year four 9.80%
Average annual percentage payout, year five 8.30%
Average annual percentage payout, year six 9.50%
Average annual percentage payout, year seven 4.90%
Average annual percentage payout, year eight 3.90%
Average annual percentage payout, year nine 2.90%
Average annual percentage payout, year ten 3.40%