Claim and Claim Adjustment Expense Reserves Short-Duration Contracts - Historical Claims Duration (Details)	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	Dec. 31, 2007
Specialty - Medical Professional Liability [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One										3.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two									18.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three								22.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four							19.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five						12.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six					8.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven				5.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight			4.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine		3.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.30%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	97.70%
Specialty - Other Professional and Management Liability [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One										5.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two									20.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three								21.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four							17.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five						10.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six					6.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven				5.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight			3.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine		1.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	3.20%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	95.10%
Specialty - Surety [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One										23.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two									32.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three								20.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four							8.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five						3.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six					1.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven				(0.70%)
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight			2.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine		1.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	2.00%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	94.80%
Commercial - Auto [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One										27.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two									23.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three								18.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four							13.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five						9.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six					3.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven				1.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight			0.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine		0.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.30%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	97.60%
Commercial - General Liability [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One										5.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two									16.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three								20.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four							20.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five						15.30%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six					8.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven				4.20%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight			2.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine		1.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	1.30%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	94.50%
Commercial - Workers Compensation [Member]
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									21.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three								14.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four							10.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Five						7.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Six					5.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven				4.60%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight			3.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine		2.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	0.30%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	83.20%
International Excluding Hardy [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One										19.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two									27.40%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three								12.10%
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					4.50%
Short-duration Insurance Contracts, Historical Claims Duration, Year Seven				4.80%
Short-duration Insurance Contracts, Historical Claims Duration, Year Eight			3.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Nine		5.00%
Short-duration Insurance Contracts, Historical Claims Duration, Year Ten	1.30%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	91.00%
Hardy [Member]
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Short-duration Insurance Contracts, Historical Claims Duration, Year One										25.70%
Short-duration Insurance Contracts, Historical Claims Duration, Year Two									40.90%
Short-duration Insurance Contracts, Historical Claims Duration, Year Three								12.10%
Short-duration Insurance Contracts, Historical Claims Duration, Year Four							4.40%
Short-Duration Insurance Contracts, Historical Claims Duration, Net	83.10%