Liability for Unpaid Loss and Loss Adjustment Expense (Details 2) (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 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
All other net loss and loss adjustment expense	330,084		329,908	984,126	982,993
Net loss and loss adjustment expense	320,376		304,014	992,547	969,767
U.S. Property & Casualty [Member]
Segment Reporting Information [Line Items]
Net (Favorable) Adverse Loss Development	(40,754)		2,138	(40,754)	2,138
Net loss and loss adjustment expense	13,666		53,229	121,060	154,156
Professional Liability [Member]
Segment Reporting Information [Line Items]
Net (Favorable) Adverse Loss Development	(26,284)		(26,186)	(26,284)	(26,186)
Net loss and loss adjustment expense	30,100		36,183	141,921	170,506
Accident & Health [Member]
Segment Reporting Information [Line Items]
Net (Favorable) Adverse Loss Development	0		(10,695)	0	(10,695)
Net loss and loss adjustment expense	163,143		140,344	483,709	447,262
U.S. Surety & Credit [Member]
Segment Reporting Information [Line Items]
Net (Favorable) Adverse Loss Development	0	(9,500)	0	(9,492)	0
Net loss and loss adjustment expense	13,436		15,721	32,287	42,444
International [Member]
Segment Reporting Information [Line Items]
Net (Favorable) Adverse Loss Development	39,196	(2,300)	0	36,896	0
Net loss and loss adjustment expense	99,221		46,924	204,137	126,547
Exited Lines [Member]
Segment Reporting Information [Line Items]
Net (Favorable) Adverse Loss Development	$ 0		$ 111	$ 0	$ 111