Loss and Loss Expense Reserves, Average Historical Claims Duration (Details)	12 Months Ended
	Dec. 31, 2018 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	17.30%
Year Two	26.30%
Year Three	16.30%
Year Four	10.40%
Year Five	5.70%
Year Six	4.30%
Year Seven	3.30%
Year Eight	1.60%
Year Nine	1.80%
Year Ten	1.30%
Physical Damage [Member]
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance [Abstract]
Year One	80.60%
Year Two	14.10%
Year Three	2.00%
Commercial Liability [Member]
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance [Abstract]
Year One	8.50%
Year Two	30.50%
Year Three	23.70%
Year Four	20.60%
Year Five	9.90%
Year Six	1.90%
Year Seven	1.90%
Year Eight	0.50%
Year Nine	0.50%
Year Ten	0.70%
Professional Liability Assumed [Member]
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance [Abstract]
Year One	1.10%
Year Two	3.60%
Year Three	13.50%
Year Four	19.00%
Year Five	18.30%
Year Six	17.80%
Year Seven	11.00%
Year Eight	1.70%
Year Nine	0.00%