Acquisition and Disposals - Catlin - Fair value of assets and liabilities acquired (Narrative) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
	May 01, 2015	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Business Acquisition [Line Items]
Goodwill				$ 14,084
Indefinite-lived intangible assets acquired during the period				8,000
Finite-lived intangible assets acquired				$ 6,000
Catlin Group Limited
Business Acquisition [Line Items]
Goodwill	$ 794,000
Indefinite-lived intangible assets acquired during the period	673,000
Finite-lived intangible assets acquired	315,000
Amortization period of value of business acquired				2 years
Amortization expense of value of business acquired		$ 39,800	$ 183,300	$ 99,400	$ 183,300
Amortization period of adjustment to unpaid losses and loss adjustment expenses				20 years
Catlin Group Limited | Insurance Segment
Business Acquisition [Line Items]
Goodwill	466,100
Catlin Group Limited | Reinsurance Segment
Business Acquisition [Line Items]
Goodwill	$ 327,900