Segment Information (Distribution of gross and net written premium) (Details)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
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%
Asia, Australia and New Zealand
Distribution Of Gross Premiums Written [Line Items]
Distribution of gross premiums written	11.00%	11.00%	11.00%
Europe
Distribution Of Gross Premiums Written [Line Items]
Distribution of gross premiums written	40.00%	40.00%	41.00%
Latin America, Caribbean and Africa
Distribution Of Gross Premiums Written [Line Items]
Distribution of gross premiums written	10.00%	10.00%	11.00%
North America
Distribution Of Gross Premiums Written [Line Items]
Distribution of gross premiums written	39.00%	39.00%	37.00%
Non Life [Member] | Property and Casualty [Member] | Casualty [Member]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	12.00%	12.00%	13.00%
Non Life [Member] | Property and Casualty [Member] | Motor [Member]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	7.00%	7.00%	5.00%
Non Life [Member] | Property and Casualty [Member] | Multiline and other [Member]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	5.00%	4.00%	3.00%
Non Life [Member] | Property and Casualty [Member] | Property [Member]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	11.00%	12.00%	14.00%
Non Life [Member] | Specialty [Member] | Agriculture [Member]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	12.00%	11.00%	7.00%
Non Life [Member] | Specialty [Member] | Aviation/Space [Member]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	4.00%	4.00%	5.00%
Non Life [Member] | Specialty [Member] | Catastrophe Line Of Business [Member]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	6.00%	8.00%	10.00%
Non Life [Member] | Specialty [Member] | Credit Surety [Member]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	7.00%	6.00%	7.00%
Non Life [Member] | Specialty [Member] | Energy [Member]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	1.00%	2.00%	2.00%
Non Life [Member] | Specialty [Member] | Engineering [Member]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	3.00%	4.00%	4.00%
Non Life [Member] | Specialty [Member] | Marine [Member]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	5.00%	6.00%	7.00%
Non Life [Member] | Specialty [Member] | Specialty casualty [Member]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	3.00%	3.00%	2.00%
Non Life [Member] | Specialty [Member] | Specialty property [Member]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	3.00%	3.00%	4.00%
Life and Health [Member]
Distribution Of Net Premiums Written [Line Items]
Distribution of net premiums written	21.00%	18.00%	17.00%