Claim, Claim Adjustment Expense and Future Policy Benefit Reserves, Supplementary Information about Average Historical Claims Duration (Details)	Dec. 31, 2020
Medical Professional Liability [Member]
Average Annual percentage Payout of Ultimate Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Year 1	3.70%
Year 2	19.50%
Year 3	24.00%
Year 4	18.40%
Year 5	12.20%
Year 6	8.20%
Year 7	5.20%
Year 8	3.10%
Year 9	0.90%
Year 10	4.10%
Other Professional Liability and Management Liability [Member]
Average Annual percentage Payout of Ultimate Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Year 1	7.00%
Year 2	22.90%
Year 3	21.40%
Year 4	16.30%
Year 5	10.10%
Year 6	6.60%
Year 7	4.40%
Year 8	2.30%
Year 9	3.00%
Year 10	2.60%
Surety [Member]
Average Annual percentage Payout of Ultimate Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Year 1	18.30%	[1]
Year 2	44.60%	[1]
Year 3	20.00%	[1]
Year 4	3.50%	[1]
Year 5	2.20%	[1]
Year 6	1.20%	[1]
Year 7	(0.70%)	[1]
Year 8	0.90%	[1]
Year 9	(1.40%)	[1]
Commercial Auto [Member]
Average Annual percentage Payout of Ultimate Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Year 1	27.90%
Year 2	24.00%
Year 3	18.30%
Year 4	14.00%
Year 5	9.80%
Year 6	2.90%
Year 7	1.30%
Year 8	0.50%
Year 10	0.30%
General Liability [Member]
Average Annual percentage Payout of Ultimate Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Year 1	4.20%
Year 2	15.00%
Year 3	18.90%
Year 4	20.30%
Year 5	14.30%
Year 6	9.10%
Year 7	5.70%
Year 8	3.20%
Year 9	2.60%
Year 10	0.30%
Workers' Compensation [Member]
Average Annual percentage Payout of Ultimate Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Year 1	14.50%
Year 2	21.90%
Year 3	14.20%
Year 4	10.10%
Year 5	6.40%
Year 6	3.50%
Year 7	2.10%
Year 8	1.10%
Year 9	0.90%
Year 10	0.30%

[1] Due to the nature of the Surety business, average annual percentage payout of ultimate net incurred claim and allocated claim adjustment expenses has been calculated using only the payouts of mature accident years presented in the loss reserve development tables.