Goodwill and Other Intangible Assets - Narrative (Details) - USD ($) $ in Thousands						12 Months Ended
		Sep. 30, 2016	Feb. 01, 2016	Jul. 24, 2015	May 01, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Goodwill [Line Items]
Finite-Lived Intangible Assets, Gross						$ 347,200	$ 333,500
Finite-Lived Intangible Assets, Accumulated Amortization						39,800	17,400
Finite-lived Intangible Assets Acquired						28,948	315,000	$ 18,500
Indefinite-lived Intangible Assets Acquired						8,000	673,000	0
Goodwill						1,213,524	1,213,630	415,936	$ 396,245
Goodwill, acquired during period						14,084	807,381	25,159
Gross goodwill						2,600,000	2,600,000
Accumulated impairment loss on goodwill						1,400,000	1,400,000
Indefinite-Lived Intangible Assets (Excluding Goodwill)						682,779	682,859	$ 15,366	$ 15,366
Insurance Segment
Goodwill [Line Items]
Goodwill						483,600	479,500
Reinsurance Segment
Goodwill [Line Items]
Goodwill						$ 730,000	$ 734,200
Allied International Holdings, Inc.
Goodwill [Line Items]
Finite-lived Intangible Assets Acquired			$ 6,000
Indefinite-lived Intangible Assets Acquired			8,000
Goodwill, acquired during period			14,100
Allied International Holdings, Inc. | Insurance Segment
Goodwill [Line Items]
Goodwill, acquired during period			$ 14,100
Catlin Group Limited [Member]
Goodwill [Line Items]
Finite-lived Intangible Assets Acquired					$ 315,000
Indefinite-lived Intangible Assets Acquired					673,000
Goodwill, acquired during period	[1]				793,990
Catlin Group Limited [Member] | Insurance Segment
Goodwill [Line Items]
Goodwill, acquired during period	[1]				466,100
Catlin Group Limited [Member] | Reinsurance Segment
Goodwill [Line Items]
Goodwill, acquired during period	[1]				$ 327,900
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

[1] ncludes an increase of $15.9 million to goodwill from the amount initially recorded at June 30, 2015, as a result of several tax-related adjustments, including a decrease in other assets of $7.5 million, a decrease in deferred tax liability of $11.3 million, and an increase in other liabilities of $19.8 million.