Goodwill and Other Intangible Assets - Intangible Assets Acquired - Catlin (Details) - USD ($) $ in Thousands		12 Months Ended
	May 01, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Acquired Intangible Assets By Major Class [Line Items]
Indefinite-lived Intangible Assets Acquired		$ 8,000	$ 673,000	$ 0
Finite-lived Intangible Assets Acquired		$ 28,948	$ 315,000	$ 18,500
Catlin Group Limited [Member]
Schedule Of Acquired Intangible Assets By Major Class [Line Items]
Indefinite-lived Intangible Assets Acquired	$ 673,000
Finite-lived Intangible Assets Acquired	315,000
Intangible Assets Acquired	988,000
Catlin Group Limited [Member] | Trademarks and Trade Names [Member]
Schedule Of Acquired Intangible Assets By Major Class [Line Items]
Finite-lived Intangible Assets Acquired	10,000
Finite-Lived Intangible Asset, Useful Life		2 years
Catlin Group Limited [Member] | Distribution Rights [Member]
Schedule Of Acquired Intangible Assets By Major Class [Line Items]
Finite-lived Intangible Assets Acquired	290,000
Finite-Lived Intangible Asset, Useful Life		20 years
Catlin Group Limited [Member] | Managing Agent Contracts [Member]
Schedule Of Acquired Intangible Assets By Major Class [Line Items]
Finite-lived Intangible Assets Acquired	15,000
Finite-Lived Intangible Asset, Useful Life		15 years
Insurance Licenses [Member] | Catlin Group Limited [Member]
Schedule Of Acquired Intangible Assets By Major Class [Line Items]
Indefinite-lived Intangible Assets Acquired	13,000
Syndicate Capacity [Member] | Catlin Group Limited [Member]
Schedule Of Acquired Intangible Assets By Major Class [Line Items]
Indefinite-lived Intangible Assets Acquired	$ 660,000