Goodwill and Intangible Assets (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Finite-Lived Intangible Assets, Future Amortization Expense [Abstract]
Future Amortization Expense Year One	$ 27,179								$ 27,179
Future Amortization Expense Year Two	27,486								27,486
Future Amortization Expense Year Three	26,593								26,593
Future Amortization Expense Year Four	25,919								25,919
Future Amortization Expense Year Five	22,818								22,818
Definite-lived intangible assets future amortization expense	129,995								129,995
Goodwill And Intangible Assets [Line Items]
Beginning balance				589,400				634,248	589,400	634,248
Acquired during the period									113,049
Amortization of intangible assets	5,100	8,900	8,900	8,900	8,900	9,500	9,200	8,800	31,799	36,405	31,461
Aggregate Amortization Of Intangible Assets									(31,799)	(44,848)
Ending balance	670,650				589,400				670,650	589,400	634,248
Intangible Assets [Line Items]
Accumulated amortization	129,178				97,379				129,178	97,379
Gross intangible assets excluding goodwill	343,448				231,246				343,448	231,246
Goodwill [Line Items]
Goodwill	456,380				455,533				456,380	455,533
[LifeMember]
Goodwill [Line Items]
Goodwill	18,804				17,957				18,804	17,957
[NorthAmericaMember]
Goodwill [Line Items]
Goodwill	82,026				82,026				82,026	82,026
[GlobalNonUsPcMember]
Goodwill [Line Items]
Goodwill	149,895				149,895				149,895	149,895
[GlobalNonUsSpecialtyMember]
Goodwill [Line Items]
Goodwill	179,641				179,641				179,641	179,641
[CatastropheSegmentMember]
Goodwill [Line Items]
Goodwill	26,014				26,014				26,014	26,014
[GoodwillMember]
Goodwill And Intangible Assets [Line Items]
Beginning balance				455,533				455,533	455,533	455,533
Acquired during the period									847
Aggregate Amortization Of Intangible Assets									0	0
Ending balance	456,380				455,533				456,380	455,533
[GoodwillMember] | [PresidioMember]
Intangible Assets [Line Items]
Definite-lived intangible assets, gross	800								800
[DefiniteLivedIntangibleAssetsMember]
Goodwill And Intangible Assets [Line Items]
Beginning balance				126,517				171,365	126,517	171,365
Acquired during the period									112,202
Aggregate Amortization Of Intangible Assets									(31,799)	(44,848)	(68,600)
Ending balance	206,920				126,517				206,920	126,517	171,365
Intangible Assets [Line Items]
Definite-lived intangible assets, gross	336,098				223,896				336,098	223,896
Accumulated amortization	129,178				97,379				129,178	97,379
[DefiniteLivedIntangibleAssetsMember] | [UnpaidLossesAndLossExpensesMember]
Intangible Assets [Line Items]
Definite-lived intangible assets, gross	191,196				191,196				191,196	191,196
Accumulated amortization	96,478				72,854				96,478	72,854
[DefiniteLivedIntangibleAssetsMember] | [RenewalRightsMember]
Intangible Assets [Line Items]
Definite-lived intangible assets, gross	80,863				32,700				80,863	32,700
Accumulated amortization	32,700				24,525				32,700	24,525
[DefiniteLivedIntangibleAssetsMember] | [RenewalRightsMember] | [PresidioMember]
Intangible Assets [Line Items]
Definite-lived intangible assets, gross	48,200								48,200
Goodwill And Intangible Assets Other Details [Abstract]
Finite Lived Intangible Assets Amortization Period									13 years 0 months 0 days
[DefiniteLivedIntangibleAssetsMember] | [CustomerRelationshipsMember]
Intangible Assets [Line Items]
Definite-lived intangible assets, gross	63,408				0				63,408	0
Accumulated amortization	0				0				0	0
[DefiniteLivedIntangibleAssetsMember] | [CustomerRelationshipsMember] | [PresidioMember]
Intangible Assets [Line Items]
Definite-lived intangible assets, gross	63,400								63,400
Goodwill And Intangible Assets Other Details [Abstract]
Finite Lived Intangible Assets Amortization Period									13 years 0 months 0 days
[DefiniteLivedIntangibleAssetsMember] | [FrontingArrangementsMember]
Intangible Assets [Line Items]
Definite-lived intangible assets, gross	631				0				631	0
Accumulated amortization	0				0				0	0
[DefiniteLivedIntangibleAssetsMember] | [FrontingArrangementsMember] | [PresidioMember]
Intangible Assets [Line Items]
Definite-lived intangible assets, gross	600								600
Goodwill And Intangible Assets Other Details [Abstract]
Finite Lived Intangible Assets Amortization Period									1 year 0 months 0 days
[IndefiniteLivedIntangibleAssetsMember]
Goodwill And Intangible Assets [Line Items]
Beginning balance				7,350				7,350	7,350	7,350
Acquired during the period									0
Aggregate Amortization Of Intangible Assets									0	0
Ending balance	7,350				7,350				7,350	7,350
[IndefiniteLivedIntangibleAssetsMember] | [InsuranceLicensesMember]
Intangible Assets [Line Items]
Accumulated amortization	0				0				0	0
Indefinite lived intangible assets	$ 7,350				$ 7,350				$ 7,350	$ 7,350