Segment Information (Net Premiums Earned By Geographic Region) (Details)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment Geographical North America [Member]
Segment Reporting [Line Items]
Percentage of net premiums earned by geographic region	61.00%	61.00%	63.00%
Segment Geographical Europe [Member]
Segment Reporting [Line Items]
Percentage of net premiums earned by geographic region	18.00%	20.00%	20.00%
Segment Geographical Asia Pacific Or Far East [Member]
Segment Reporting [Line Items]
Percentage of net premiums earned by geographic region	14.00%	13.00%	12.00%
Segment Geographical Latin America [Member]
Segment Reporting [Line Items]
Percentage of net premiums earned by geographic region	7.00%	6.00%	5.00%