Goodwill and Intangible Assets (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Finite-Lived Intangible Assets, Future Amortization Expense [Abstract]
2014	$ 27,486
2015	26,593
2016	25,919
2017	22,818
2018	21,247
Total	124,063
Intangible Assets [Line Items]
Gross intangible assets excluding goodwill	343,448	343,448
Accumulated amortization	156,358	129,178
Goodwill [Line Items]
Goodwill	456,380	456,380
North America Segment [Member]
Goodwill [Line Items]
Goodwill	82,026	82,026
Global (Non-US) P&C [Member]
Goodwill [Line Items]
Goodwill	149,895	149,895
Global Specialty [Member]
Goodwill [Line Items]
Goodwill	179,641	179,641
Catastrophe Segment [Member]
Goodwill [Line Items]
Goodwill	26,014	26,014
Life and Health [Member]
Goodwill [Line Items]
Goodwill	18,804	18,804
Goodwill [Member]
Goodwill And Intangible Assets [Line Items]
Balance at January 1	456,380	455,533
Acquired during the year		847
Intangible assets amortization	0	0
Balance at December 31	456,380	456,380
Definite Lived Intangible Assets [Member]
Goodwill And Intangible Assets [Line Items]
Balance at January 1	206,920	126,517
Acquired during the year		112,202
Intangible assets amortization	(27,180)	(31,799)	(44,800)
Balance at December 31	179,740	206,920	126,517
Intangible Assets [Line Items]
Definite-lived intangible assets, gross	336,098	336,098
Accumulated amortization	156,358	129,178
Definite Lived Intangible Assets [Member] | Unpaid Losses And Loss Expenses [Member]
Intangible Assets [Line Items]
Definite-lived intangible assets, gross	191,196	191,196
Accumulated amortization	115,958	96,478
Definite Lived Intangible Assets [Member] | Renewal Rights [Member]
Intangible Assets [Line Items]
Definite-lived intangible assets, gross	80,863	80,863
Accumulated amortization	38,132	32,700
Definite Lived Intangible Assets [Member] | Customer Relationships [Member]
Intangible Assets [Line Items]
Definite-lived intangible assets, gross	63,408	63,408
Accumulated amortization	1,637	0
Definite Lived Intangible Assets [Member] | Fronting Arrangements [Member]
Intangible Assets [Line Items]
Definite-lived intangible assets, gross	631	631
Accumulated amortization	631	0
Indefinite Lived Intangible Assets [Member]
Goodwill And Intangible Assets [Line Items]
Balance at January 1	7,350	7,350
Acquired during the year		0
Intangible assets amortization	0	0
Balance at December 31	7,350	7,350
Indefinite Lived Intangible Assets [Member] | Insurance Licenses [Member]
Intangible Assets [Line Items]
Indefinite lived intangible assets	7,350	7,350
Total intangible assets [Member]
Goodwill And Intangible Assets [Line Items]
Balance at January 1	214,270	133,867
Acquired during the year		112,202
Intangible assets amortization	(27,180)	(31,799)
Balance at December 31	$ 187,090	$ 214,270