Segment Information (Broker details) (Details)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Economic Dependence [Line Items]
Cedant Percentage Of Gross Written Premium	3.00%	4.00%	4.00%
North America Segment [Member]
Economic Dependence [Line Items]
Broker Percentage Of Gross Written Premium	63.00%	59.00%	60.00%
Global (Non-US) P&C [Member]
Economic Dependence [Line Items]
Broker Percentage Of Gross Written Premium	28.00%	31.00%	29.00%
Global Specialty [Member]
Economic Dependence [Line Items]
Broker Percentage Of Gross Written Premium	38.00%	38.00%	41.00%
Catastrophe Segment [Member]
Economic Dependence [Line Items]
Broker Percentage Of Gross Written Premium	75.00%	70.00%	74.00%
Life and Health [Member]
Economic Dependence [Line Items]
Broker Percentage Of Gross Written Premium	16.00%	12.00%	12.00%
Broker A [Member]
Economic Dependence [Line Items]
Broker Percentage Of Gross Written Premium	19.20%	20.00%	22.00%
Broker B [Member]
Economic Dependence [Line Items]
Broker Percentage Of Gross Written Premium	22.10%	20.00%	21.00%