Segment Information (Net Premiums Earned by Line of Business) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Segment Reporting Information [Line Items]
Net premiums earned	$ 650,653	$ 541,737	$ 1,865,577	$ 1,609,269
Operating Segments [Member] | North American Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	332,722	277,203	974,232	808,453
Operating Segments [Member] | North American Insurance [Member] | Casualty [Member]
Segment Reporting Information [Line Items]
Net premiums earned	112,093	86,114	326,236	240,981
Operating Segments [Member] | North American Insurance [Member] | Professional Liability [Member]
Segment Reporting Information [Line Items]
Net premiums earned	87,678	68,512	251,005	197,042
Operating Segments [Member] | North American Insurance [Member] | Property [Member]
Segment Reporting Information [Line Items]
Net premiums earned	39,333	42,457	123,201	127,402
Operating Segments [Member] | North American Insurance [Member] | Programs [Member]
Segment Reporting Information [Line Items]
Net premiums earned	38,306	28,933	109,163	88,109
Operating Segments [Member] | North American Insurance [Member] | Healthcare [Member]
Segment Reporting Information [Line Items]
Net premiums earned	35,322	40,775	111,759	126,591
Operating Segments [Member] | North American Insurance [Member] | Specialty and Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	19,990	10,412	52,868	28,328
Operating Segments [Member] | Global Markets Insurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	110,327	41,574	262,993	114,584
Operating Segments [Member] | Global Markets Insurance [Member] | Casualty [Member]
Segment Reporting Information [Line Items]
Net premiums earned	32,033	4,396	68,979	10,298
Operating Segments [Member] | Global Markets Insurance [Member] | Professional Liability [Member]
Segment Reporting Information [Line Items]
Net premiums earned	27,719	17,001	76,976	47,091
Operating Segments [Member] | Global Markets Insurance [Member] | Property [Member]
Segment Reporting Information [Line Items]
Net premiums earned	20,623	8,262	47,152	25,260
Operating Segments [Member] | Global Markets Insurance [Member] | Specialty and Other [Member]
Segment Reporting Information [Line Items]
Net premiums earned	29,952	11,915	69,886	31,935
Operating Segments [Member] | Reinsurance [Member]
Segment Reporting Information [Line Items]
Net premiums earned	207,604	222,960	628,352	686,232
Operating Segments [Member] | Reinsurance [Member] | Property [Member]
Segment Reporting Information [Line Items]
Net premiums earned	102,329	113,560	311,498	379,575
Operating Segments [Member] | Reinsurance [Member] | Casualty [Member]
Segment Reporting Information [Line Items]
Net premiums earned	58,170	65,915	173,953	163,591
Operating Segments [Member] | Reinsurance [Member] | Specialty [Member]
Segment Reporting Information [Line Items]
Net premiums earned	$ 47,105	$ 43,485	$ 142,901	$ 143,066