Claim and Claim Adjustment Expense Reserves (Historical Claims Duration) (Details)	Dec. 31, 2018
Medical Professional Liability | Specialty
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	3.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	19.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	24.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	20.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	12.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	7.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	4.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	2.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	1.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	2.90%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	98.10%
Other Professional Liability and Management Liability | Specialty
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	6.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	21.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	21.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	17.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	10.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	6.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	5.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	2.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	3.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	2.10%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	97.60%
Surety | Specialty
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	21.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	37.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	21.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	7.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	2.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	1.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	(1.70%)
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	(0.50%)
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.00%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	89.90%
Commercial Auto | Commercial
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	28.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	23.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	18.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	14.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	9.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	3.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	1.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	0.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	0.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.70%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	98.60%
General Liability | Commercial
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	4.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	15.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	19.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	20.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	15.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	8.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	5.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	3.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	1.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.80%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	95.10%
Workers' Compensation | Commercial
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	13.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	21.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	13.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	10.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	7.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	4.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	4.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	3.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	2.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	1.20%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	82.50%
Excluding Hardy | International
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	20.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	26.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	11.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	7.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	6.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	4.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven	4.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight	2.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine	1.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	3.70%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	88.60%
Hardy | International
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One	24.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two	38.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three	13.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four	6.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five	2.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six	2.10%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	87.20%