Supplementary Information of Average Historical Claims (Parenthetical) (Detail) - Reinsurance Operations - Casualty Insurance	Dec. 31, 2020
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average annual percentage payout of incurred claims by age, net of reinsurance year one	408.40%	[1],[2]
Average annual percentage payout of incurred claims by age, net of reinsurance year two	(204.00%)	[1],[2]
Average annual percentage payout of incurred claims by age, net of reinsurance year three	23.80%	[1],[2]
Average annual percentage payout of incurred claims by age, net of reinsurance year four	(305.20%)	[1],[2]
Average annual percentage payout of incurred claims by age, net of reinsurance year five	(1.20%)	[1],[2]
Average annual percentage payout of incurred claims by age, net of reinsurance year six	0.60%	[1],[2]
Average annual percentage payout of incurred claims by age, net of reinsurance year seven	1.50%	[1],[2]
Average annual percentage payout of incurred claims by age, net of reinsurance year eight	(0.10%)	[1],[2]
Average annual percentage payout of incurred claims by age, net of reinsurance year nine	0.40%	[1],[2]
Average annual percentage payout of incurred claims by age, net of reinsurance year ten	0.50%	[1],[2]
Weighted Average
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Average annual percentage payout of incurred claims by age, net of reinsurance year one	11.70%
Average annual percentage payout of incurred claims by age, net of reinsurance year two	19.50%
Average annual percentage payout of incurred claims by age, net of reinsurance year three	14.40%
Average annual percentage payout of incurred claims by age, net of reinsurance year four	16.90%
Average annual percentage payout of incurred claims by age, net of reinsurance year five	4.80%
Average annual percentage payout of incurred claims by age, net of reinsurance year six	1.70%
Average annual percentage payout of incurred claims by age, net of reinsurance year seven	0.20%
Average annual percentage payout of incurred claims by age, net of reinsurance year eight	0.50%
Average annual percentage payout of incurred claims by age, net of reinsurance year nine	0.40%
Average annual percentage payout of incurred claims by age, net of reinsurance year ten	0.50%

[1] May not be indicative of future average annual percentage payout of incurred claims due to a change in mix of business

[2] The payout patterns are calculated using simple averages consistent with last year’s calculation methodology. However, these payout patterns based on simple averages look unusual given the change in estimate of ultimate losses for accident years 2013 through 2015 during the year and due to very minimal yet volatile payment activity for those accident years. Using a weighted average approach would produce smoother payout patterns as less weight is given to the accident years with minimal paid losses. The resulting volume weighted ten year annual payout pattern, for ages one through ten would be 11.7%, 19.5%, 14.4%, 16.9%, 4.8%, 1.7%, 0.2%, 0.5%, 0.4%, and 0.5%, respectively.