Claim and Claim Adjustment Expense Reserves (Historical Claims Duration) (Details)	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008
Medical Professional Liability | Specialty
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One										3.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two									18.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three								23.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four							19.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five						12.70%
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			3.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine		3.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.70%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	97.50%
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										5.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two									21.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three								21.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four							17.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five						9.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six					6.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven				5.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight			4.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine		2.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	1.50%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	94.90%
Surety | Specialty
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One										21.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two									38.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three								17.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four							7.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five						3.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six					2.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven				(2.30%)
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight			(1.10%)
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	87.00%
Commercial Auto | Commercial
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One										28.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two									22.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three								18.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four							14.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five						9.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six					3.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven				0.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight			0.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine		0.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.70%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	98.30%
General Liability | Commercial
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One										4.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two									16.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three								20.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four							20.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five						15.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six					8.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven				4.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight			2.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine		1.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	1.20%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	96.00%
Workers' Compensation | Commercial
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One										13.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two									21.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three								14.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four							10.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five						7.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six					5.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven				4.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight			3.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine		3.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	3.20%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	87.90%
Excluding Hardy | International
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One										19.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two									25.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three								12.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four							6.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five						5.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six					4.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven				5.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight			3.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine		5.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.40%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	89.20%
Hardy | International
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One										24.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two									39.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three								14.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four							5.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five						2.80%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	85.80%