Liability for Unpaid Loss and Loss Adjustment Expense (Details Textuals1) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development	$ (56,357)	$ (73,742)	$ (70,011)
U.S. Property & Casualty [Member]
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development	(41,560)	(39,363)	2,321
Professional Liability [Member]
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development	973	(26,346)	(25,897)
Accident & Health [Member]
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development	(13,086)	(18,027)	(10,511)
U.S. Surety & Credit [Member]
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development	(22,629)	(37,898)	(25,377)
International [Member]
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development	25,050	43,805	(10,084)
Exited Lines [Member]
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development	(5,105)	4,087	(463)
Aviation [Member] | U.S. Property & Casualty [Member]
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development	9,000
E & O [Member] | U.S. Property & Casualty [Member]
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development		5,800	7,000
Public Risk [Member] | U.S. Property & Casualty [Member]
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development			8,100
Surety [Member] | U.S. Surety & Credit [Member]
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development	20,300	20,600	18,000
Credit LOB [Member] | U.S. Surety & Credit [Member]
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development	2,300	17,300	7,400
Energy [Member] | International [Member]
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development		10,100
Liability LOB [Member] | U.S. Property & Casualty [Member]
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development	18,700
Liability LOB [Member] | International [Member]
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development		16,100
Surety & Credit [Member] | International [Member]
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development	44,100	70,300
D&O prior to 2007 and 2009 and 2010 | Professional Liability [Member]
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development		(64,200)	62,100
D&O 2007 and 2008 | Professional Liability [Member]
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development		37,900
International segment - release of 2010 catastrophe reserves for New Zealand earthquake | International [Member]
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development		(2,300)
Superstorm Sandy [Member] | International [Member]
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development		3,400
Hurricane Irene [Member] | International [Member]
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development			10,700
Japan Earthquake and Tsunami [Member] | International [Member]
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development			4,600
Spanish Surety Bonds [Member] | International [Member]
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development			48,900
Quota Share Contract Member [Member] | U.S. Property & Casualty [Member]
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development	5,000	17,000	5,600
Prior Year Catastrophe Reserve Releases [Member]
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development	5,900	7,300	21,400
Prior Year Catastrophe Reserve Releases [Member] | International [Member]
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development	5,400		18,900
Liability And Energy Lines [Member] | International [Member]
Segment LOB Event Reporting Information [Line Items]
Total net (favorable) adverse loss development			$ 59,000