Segment Information (Broker details) (Details)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Economic Dependence [Line Items]
Cedant Percentage Of Gross Written Premium	5.00%	5.00%	5.00%
[LifeMember]
Economic Dependence [Line Items]
Broker Percentage Of Gross Written Premium	13.00%	16.00%	15.00%
[NorthAmericaMember]
Economic Dependence [Line Items]
Broker Percentage Of Gross Written Premium	70.00%	64.00%	77.00%
[GlobalNonUsPcMember]
Economic Dependence [Line Items]
Broker Percentage Of Gross Written Premium	28.00%	29.00%	32.00%
[GlobalNonUsSpecialtyMember]
Economic Dependence [Line Items]
Broker Percentage Of Gross Written Premium	42.00%	43.00%	37.00%
[CatastropheSegmentMember]
Economic Dependence [Line Items]
Broker Percentage Of Gross Written Premium	74.00%	81.00%	70.00%
[BrokerAMember]
Economic Dependence [Line Items]
Broker Percentage Of Gross Written Premium	24.00%	26.00%	25.00%
[BrokerBMember]
Economic Dependence [Line Items]
Broker Percentage Of Gross Written Premium	22.00%	21.00%	21.00%