Segments (Details 1) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Assets	$ 9,625,253	$ 9,064,082
United States Property And Casualty [Member]
Segment Reporting Information [Line Items]
Assets	949,100	924,219
Professional Liability [Member]
Segment Reporting Information [Line Items]
Assets	1,031,024	997,726
Accident And Health [Member]
Segment Reporting Information [Line Items]
Assets	236,846	238,310
United States Surety And Credit [Member]
Segment Reporting Information [Line Items]
Assets	161,968	172,562
International [Member]
Segment Reporting Information [Line Items]
Assets	728,338	609,687
Investing [Member]
Segment Reporting Information [Line Items]
Assets	6,119,288	5,748,822
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Assets	$ 398,689	$ 372,756