Claim, Claim Adjustment Expense and Future Policy Benefit Reserves, Supplementary Information about Average Historical Claims Duration (Details)	Dec. 31, 2021
Medical Professional Liability
Average Annual percentage Payout of Ultimate Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	3.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	18.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	23.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	18.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	12.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	8.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	5.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	2.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	1.00%
Other Professional Liability and Management Liability
Average Annual percentage Payout of Ultimate Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	6.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	21.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	21.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	16.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	9.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	6.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	4.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	3.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	1.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.50%
Surety
Average Annual percentage Payout of Ultimate Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	14.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	48.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	18.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	2.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	1.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	1.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	(0.80%)
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.00%
Commercial Auto
Average Annual percentage Payout of Ultimate Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	26.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	23.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	18.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	13.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	10.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	3.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	1.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.30%
General Liability
Average Annual percentage Payout of Ultimate Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	4.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	14.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	18.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	19.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	13.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	8.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	5.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	4.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	2.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.80%
Workers' Compensation
Average Annual percentage Payout of Ultimate Net Incurred Claim and Allocated Claim Adjustment Expenses [Abstract]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	14.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	21.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	13.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	9.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	6.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	2.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	1.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	1.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.50%