Acquisitions (Summary Of Fair Value Of Separately Identifiable Intangible Assets On Acquisition) (Details) (Darwin Professional Underwriters, Inc. [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011 years
Business Acquisition [Line Items]
Identifiable Intangible Assets, Fair Value	$ 56,200
Identifiable Intangible Assets, Amortization Period (in years)	14.3
Renewal Rights [Member]
Business Acquisition [Line Items]
Identifiable Intangible Assets, Fair Value	38,000
Identifiable Intangible Assets, Amortization Period (in years)	15
Trademarks [Member]
Business Acquisition [Line Items]
Identifiable Intangible Assets, Fair Value	7,400
Identifiable Intangible Assets, Amortization Period (in years)	15
Internally Developed Software[Member]
Business Acquisition [Line Items]
Identifiable Intangible Assets, Fair Value	1,600
Identifiable Intangible Assets, Amortization Period (in years)	3
Non-Compete Covenants [Member]
Business Acquisition [Line Items]
Identifiable Intangible Assets, Fair Value	1,200
Identifiable Intangible Assets, Amortization Period (in years)	2
Insurance Licenses [Member]
Business Acquisition [Line Items]
Identifiable Intangible Assets, Fair Value	$ 8,000