Goodwill and Other Intangible Assets - Narrative (Details) - USD ($) $ in Thousands				12 Months Ended
		Jul. 24, 2015	May. 01, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Goodwill [Line Items]
Finite-Lived Intangible Assets, Gross				$ 333,500	$ 18,500
Finite-Lived Intangible Assets, Accumulated Amortization				17,400	1,900
Indefinite-lived Intangible Assets Acquired				673,000
Goodwill				1,213,630	415,936	$ 396,245	$ 392,882
Goodwill, acquired during period				807,381	25,159
Gross goodwill				2,600,000	1,800,000
Accumulated impairment loss on goodwill				1,400,000
Indefinite-Lived Intangible Assets (Excluding Goodwill)				682,859	15,366	$ 15,366	$ 15,366
Insurance Segment [Member]
Goodwill [Line Items]
Goodwill				479,500
Reinsurance Segment [Member]
Goodwill [Line Items]
Goodwill				$ 734,200	$ 415,936
Catlin Group Limited [Member]
Goodwill [Line Items]
Indefinite-lived Intangible Assets Acquired			$ 673,000
Goodwill, acquired during period	[1]		793,990
Catlin Group Limited [Member] | Insurance Segment [Member]
Goodwill [Line Items]
Goodwill, acquired during period	[1]		466,100
Catlin Group Limited [Member] | Reinsurance Segment [Member]
Goodwill [Line Items]
Goodwill, acquired during period	[1]		$ 327,900
New Energy Risk Inc. [Member]
Goodwill [Line Items]
Goodwill, acquired during period		$ 13,400

[1] Includes 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.