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Goodwill and Other Intangible Assets (Narrative) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Mar. 02, 2015
Minimum        
Goodwill and Intangible Assets [Line Items]        
Finite-lived intangible assets, useful life 1 year      
Maximum        
Goodwill and Intangible Assets [Line Items]        
Finite-lived intangible assets, useful life 16 years      
Weighted Average        
Goodwill and Intangible Assets [Line Items]        
Finite-lived intangible assets, useful life 7 years 10 months 24 days      
Investments in other ventures, under equity method        
Goodwill and Intangible Assets [Line Items]        
Goodwill $ 7,818 $ 7,818 $ 7,818  
Platinum Underwriters Holdings, Ltd.        
Goodwill and Intangible Assets [Line Items]        
Goodwill       $ 191,729
Finite lived intangible assets acquired       75,200
Indefinite lived intangible assets acquired       $ 8,400