Segment Information (Distribution of gross and net written premium) (Details)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	100.00%	100.00%	100.00%
Distribution Of Gross Premiums Written [Line Items]
Distribution of gross premiums written	100.00%	100.00%	100.00%
[NorthAmericaGeographicMember]
Distribution Of Gross Premiums Written [Line Items]
Distribution of gross premiums written	37.00%	36.00%	36.00%
[EuropeMember]
Distribution Of Gross Premiums Written [Line Items]
Distribution of gross premiums written	41.00%	41.00%	43.00%
[LatinAmericaCaribbeanAndAfricaMember]
Distribution Of Gross Premiums Written [Line Items]
Distribution of gross premiums written	11.00%	11.00%	11.00%
[AsiaAustraliaAndNewZealandMember]
Distribution Of Gross Premiums Written [Line Items]
Distribution of gross premiums written	11.00%	12.00%	10.00%
[LifeMember]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	17.00%	18.00%	16.00%
[NonLifeMember] | [PropertyAndCasualtyMember] | [CasualtyMember]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	13.00%	11.00%	11.00%
[NonLifeMember] | [PropertyAndCasualtyMember] | [PropertyMember]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	14.00%	15.00%	18.00%
[NonLifeMember] | [PropertyAndCasualtyMember] | [MotorMember]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	5.00%	5.00%	7.00%
[NonLifeMember] | [PropertyAndCasualtyMember] | [MultilineMember]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	3.00%	2.00%	2.00%
[NonLifeMember] | [SpecialtyMember] | [AgricultureMember]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	7.00%	7.00%	4.00%
[NonLifeMember] | [SpecialtyMember] | [AviationspaceMember]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	5.00%	5.00%	5.00%
[NonLifeMember] | [SpecialtyMember] | [CatastropheLineOfBusinessMember]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	10.00%	13.00%	14.00%
[NonLifeMember] | [SpecialtyMember] | [CreditSuretyMember]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	7.00%	7.00%	6.00%
[NonLifeMember] | [SpecialtyMember] | [EngineeringMember]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	4.00%	4.00%	4.00%
[NonLifeMember] | [SpecialtyMember] | [EnergyLineOfBusinessMember]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	2.00%	2.00%	2.00%
[NonLifeMember] | [SpecialtyMember] | [MarineMember]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	7.00%	6.00%	6.00%
[NonLifeMember] | [SpecialtyMember] | [SpecialtyCasualtyMember]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	2.00%	2.00%	3.00%
[NonLifeMember] | [SpecialtyMember] | [SpecialtyPropertyMember]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	4.00%	3.00%	2.00%