Loss and Loss Expense Reserves, Average Historical Claims Duration (Details)	12 Months Ended
	Dec. 31, 2019 Quarter yr
Minimum [Member]
Reserve Methodologies for Incurred But Not Reported Losses [Abstract]
Number of running accident quarters used | Quarter	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	17.50%
Year Two	26.40%
Year Three	16.10%
Year Four	10.30%
Year Five	6.20%
Year Six	4.00%
Year Seven	3.40%
Year Eight	1.50%
Year Nine	1.40%
Year Ten	1.40%
Physical Damage [Member]
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance [Abstract]
Year One	81.50%
Year Two	15.60%
Year Three	2.00%
Commercial Liability [Member]
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance [Abstract]
Year One	9.00%
Year Two	25.80%
Year Three	23.50%
Year Four	21.00%
Year Five	10.40%
Year Six	4.70%
Year Seven	3.50%
Year Eight	0.40%
Year Nine	1.50%
Year Ten	0.00%
Professional Liability Assumed [Member]
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance [Abstract]
Year One	1.50%
Year Two	4.10%
Year Three	13.40%
Year Four	26.60%
Year Five	18.10%
Year Six	15.60%
Year Seven	8.80%
Year Eight	3.30%
Year Nine	4.40%
Year Ten	0.70%