Liability for Unpaid Loss and Loss Adjustment Expense (Details 2) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Total net (favorable) adverse loss development									$ (56,357)	$ (73,742)	$ (70,011)
Accident year catastrophe losses									28,683	55,939	52,390
All other net loss and loss adjustment expense									1,354,509	1,340,257	1,355,695
Loss and loss adjustment expense, net	339,434	320,758	335,897	330,746	305,477	328,530	347,353	341,094	1,326,835	1,322,454	1,338,074
U.S. Property & Casualty [Member]
Segment Reporting Information [Line Items]
Total net (favorable) adverse loss development									(41,560)	(39,363)	2,321
Loss and loss adjustment expense, net									154,200	175,190	209,286
Professional Liability [Member]
Segment Reporting Information [Line Items]
Total net (favorable) adverse loss development									973	(26,346)	(25,897)
Loss and loss adjustment expense, net									206,690	195,429	229,873
Accident & Health [Member]
Segment Reporting Information [Line Items]
Total net (favorable) adverse loss development									(13,086)	(18,027)	(10,511)
Loss and loss adjustment expense, net									696,288	630,210	601,076
U.S. Surety & Credit [Member]
Segment Reporting Information [Line Items]
Total net (favorable) adverse loss development									(22,629)	(37,898)	(25,377)
Loss and loss adjustment expense, net									42,273	24,143	38,535
International [Member]
Segment Reporting Information [Line Items]
Total net (favorable) adverse loss development									25,050	43,805	(10,084)
Loss and loss adjustment expense, net									196,134	249,199	189,410
Exited Lines [Member]
Segment Reporting Information [Line Items]
Total net (favorable) adverse loss development									$ (5,105)	$ 4,087	$ (463)