Acquisition and Disposals - Catlin - Fair value of assets and liabilities acquired (Narrative) (Details) - USD ($) $ in Thousands		3 Months Ended
	May. 01, 2015	Mar. 31, 2016
Business Acquisition [Line Items]
Goodwill		$ 13,946
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
Business Combination Measurement Period		12 months
Value of Business Acquired Adjustment, Amortization Period		2 years
Intangibles Assets Arising from Insurance Contracts Acquired in Business Combination, Amortization Expense		$ 59,600
Unpaid Losses And Loss Expenses Adjustment Amortization Period		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