Goodwill and Other Intangible Assets (Details) - USD ($) $ in Thousands					9 Months Ended
	Sep. 30, 2016	Feb. 01, 2016	Jul. 24, 2015	May 01, 2015	Sep. 30, 2016	Dec. 31, 2015
Schedule Of Acquired Intangible Assets By Major Class [Line Items]
Goodwill and other intangible assets	$ 2,234,071				$ 2,234,071	$ 2,210,266
Finite-lived intangible assets acquired					28,948
Goodwill acquired during the period					$ 14,084
Brooklyn Underwriting Pty Limited and Brooklyn IT Pty Limited [Member]
Schedule Of Acquired Intangible Assets By Major Class [Line Items]
Finite-lived intangible assets acquired	$ 22,900
Allied International Holdings, Inc.
Schedule Of Acquired Intangible Assets By Major Class [Line Items]
Finite-lived intangible assets acquired		$ 6,000
Total identified intangible assets		14,000
Goodwill acquired during the period		$ 14,100
New Energy Risk Inc.
Schedule Of Acquired Intangible Assets By Major Class [Line Items]
Goodwill acquired during the period			$ 13,400
Catlin Group Limited
Schedule Of Acquired Intangible Assets By Major Class [Line Items]
Finite-lived intangible assets acquired				$ 315,000
Total identified intangible assets				988,000
Goodwill acquired during the period				$ 794,000