Segments (Details 1) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Assets	$ 10,267,807	$ 9,597,278
United States Property And Casualty [Member]
Segment Reporting Information [Line Items]
Assets	859,597	938,265
Professional Liability [Member]
Segment Reporting Information [Line Items]
Assets	1,053,024	1,026,639
Accident And Health [Member]
Segment Reporting Information [Line Items]
Assets	243,023	236,811
United States Surety And Credit [Member]
Segment Reporting Information [Line Items]
Assets	162,817	157,037
International [Member]
Segment Reporting Information [Line Items]
Assets	658,632	722,933
Investing [Member]
Segment Reporting Information [Line Items]
Assets	7,018,747	6,119,288
Corporate & Other [Member]
Segment Reporting Information [Line Items]
Assets	$ 271,967	$ 396,305