Loss and Loss Expense Reserves, Supplementary Information about Average Historical Claims Duration (Details)	12 Months Ended
	Dec. 31, 2020 qtr yr
Minimum [Member]
Reserve Methodologies for Incurred But Not Reported Losses [Abstract]
Number of running accident quarters used | qtr	12
Number of accident years included in loss development triangles | yr	15
Workers' Compensation [Member]
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance [Abstract]
Year One	16.90%
Year Two	26.50%
Year Three	16.10%
Year Four	10.10%
Year Five	6.20%
Year Six	3.80%
Year Seven	3.10%
Year Eight	2.00%
Year Nine	1.10%
Year Ten	0.60%
Occupational Accident [Member]
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance [Abstract]
Year One	36.20%
Year Two	40.70%
Year Three	11.20%
Year Four	4.10%
Year Five	1.00%
Year Six	1.60%
Year Seven	(0.10%)
Year Eight	0.40%
Year Nine	0.60%
Year Ten	0.30%
Physical Damage [Member]
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance [Abstract]
Year One	81.60%
Year Two	16.30%
Year Three	(0.40%)
Commercial Liability [Member]
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance [Abstract]
Year One	8.80%
Year Two	25.20%
Year Three	22.80%
Year Four	19.80%
Year Five	8.90%
Year Six	5.40%
Year Seven	3.30%
Year Eight	0.80%
Year Nine	1.10%
Year Ten	0.00%
Professional Liability Assumed [Member]
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance [Abstract]
Year One	1.40%
Year Two	3.20%
Year Three	11.80%
Year Four	27.60%
Year Five	16.90%
Year Six	14.90%
Year Seven	9.00%
Year Eight	4.10%
Year Nine	2.60%
Year Ten	1.70%