Loss and Loss Expense Reserves, Average Historical Claims Duration (Details)	12 Months Ended
	Dec. 31, 2016 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	17.40%
Year Two	28.30%
Year Three	15.30%
Year Four	8.10%
Year Five	5.30%
Year Six	4.50%
Year Seven	2.90%
Year Eight	2.70%
Year Nine	1.30%
Year Ten	2.10%
Professional Liability Assumed [Member]
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance [Abstract]
Year One	1.70%
Year Two	4.60%
Year Three	17.40%
Year Four	23.70%
Year Five	21.10%
Year Six	18.20%
Year Seven	10.50%
Commercial Liability [Member]
Average Annual Percentage Payout of Incurred Claims by Age, Net of Reinsurance [Abstract]
Year One	9.70%
Year Two	31.30%
Year Three	26.70%
Year Four	20.00%
Year Five	6.20%
Year Six	2.20%
Year Seven	0.70%
Year Eight	0.20%
Year Nine	0.20%
Year Ten	0.10%