Liability for Unpaid Loss and Loss Adjustment Expense (Details Textuals1) (USD $) In Thousands, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2013	Jun. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Lob Event Reporting Information [Line Items]
Net (Favorable) Adverse Loss Development	$ (27,842)		$ (34,632)	$ (39,634)	$ (34,632)
Catastrophe losses	18,134		8,738	48,055	21,406
U.S. Property & Casualty [Member]
Segment Lob Event Reporting Information [Line Items]
Net (Favorable) Adverse Loss Development	(40,754)		2,138	(40,754)	2,138
Professional Liability [Member]
Segment Lob Event Reporting Information [Line Items]
Net (Favorable) Adverse Loss Development	(26,284)		(26,186)	(26,284)	(26,186)
Accident & Health [Member]
Segment Lob Event Reporting Information [Line Items]
Net (Favorable) Adverse Loss Development	0		(10,695)	0	(10,695)
U.S. Surety & Credit [Member]
Segment Lob Event Reporting Information [Line Items]
Net (Favorable) Adverse Loss Development	0	(9,500)	0	(9,492)	0
International [Member]
Segment Lob Event Reporting Information [Line Items]
Net (Favorable) Adverse Loss Development	39,196	(2,300)	0	36,896	0
Exited Lines [Member]
Segment Lob Event Reporting Information [Line Items]
Net (Favorable) Adverse Loss Development	0		111	0	111
US D & O [Member] | Professional Liability [Member]
Segment Lob Event Reporting Information [Line Items]
Net (Favorable) Adverse Loss Development	(9,300)			(9,300)
International D & O [Member] | Professional Liability [Member]
Segment Lob Event Reporting Information [Line Items]
Net (Favorable) Adverse Loss Development	(16,900)			(16,900)
Surety [Member] | U.S. Surety & Credit [Member]
Segment Lob Event Reporting Information [Line Items]
Net (Favorable) Adverse Loss Development		(3,700)		(3,700)
Credit LOB [Member] | U.S. Surety & Credit [Member]
Segment Lob Event Reporting Information [Line Items]
Net (Favorable) Adverse Loss Development		(5,800)		(5,800)
Energy [Member] | International [Member]
Segment Lob Event Reporting Information [Line Items]
Net (Favorable) Adverse Loss Development	(13,100)			(13,100)
Liability LOB [Member] | International [Member]
Segment Lob Event Reporting Information [Line Items]
Net (Favorable) Adverse Loss Development	(13,300)			(13,300)
Surety & Credit [Member] | International [Member]
Segment Lob Event Reporting Information [Line Items]
Net (Favorable) Adverse Loss Development	70,300			70,300
D&O prior to 2007 and 2009 and 2010 | Professional Liability [Member]
Segment Lob Event Reporting Information [Line Items]
Net (Favorable) Adverse Loss Development	(64,200)			(64,200)
D&O 2007 and 2008 | Professional Liability [Member]
Segment Lob Event Reporting Information [Line Items]
Net (Favorable) Adverse Loss Development	37,900			37,900
Energy LOB - release of 2012 catastrophe reserves for Hurricane Sandy | International [Member]
Segment Lob Event Reporting Information [Line Items]
Net (Favorable) Adverse Loss Development	(3,000)			(3,000)
International segment - release of 2010 catastrophe reserves for New Zealand earthquake | International [Member]
Segment Lob Event Reporting Information [Line Items]
Net (Favorable) Adverse Loss Development		$ (2,300)		$ (2,300)