Goodwill and Other Intangible Assets - Narrative (Details) - USD ($) $ in Thousands				12 Months Ended
	Sep. 30, 2016	Feb. 01, 2016	Jul. 24, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Goodwill [Line Items]
Goodwill				$ 1,234,150	$ 1,213,524	$ 1,213,630	$ 415,936
Goodwill, acquired during period					14,084	807,381
Gross goodwill				2,600,000	2,600,000
Accumulated impairment loss on goodwill				1,400,000	1,400,000
Finite-Lived Intangible Assets, Gross				354,200	347,200
Finite-Lived Intangible Assets, Accumulated Amortization				$ 61,600	39,800
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life				16 years 8 months 12 days
Finite-lived Intangible Assets Acquired					28,948	315,000
Indefinite-lived Intangible Assets Acquired					8,000	$ 673,000
Insurance Segment
Goodwill [Line Items]
Goodwill				$ 492,300	483,600
Reinsurance Segment
Goodwill [Line Items]
Goodwill				$ 741,900	$ 730,000
New Energy Risk Inc. [Member]
Goodwill [Line Items]
Goodwill, acquired during period			$ 13,400
Brooklyn Underwriting Pty Limited and Brooklyn IT Pty Limited [Member]
Goodwill [Line Items]
Finite-lived Intangible Assets Acquired	$ 22,900
Allied International Holdings, Inc.
Goodwill [Line Items]
Goodwill, acquired during period		$ 14,100
Finite-lived Intangible Assets Acquired		6,000
Indefinite-lived Intangible Assets Acquired		8,000
Allied International Holdings, Inc. | Insurance Segment
Goodwill [Line Items]
Goodwill, acquired during period		$ 14,100