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Investment Securities (Other long-term investments) (Details) - USD ($)
$ in Millions
Sep. 30, 2018
Dec. 31, 2017
Hedge and private equity funds included in other long-term investments    
Debt and Equity Securities, FV-NI [Line Items]    
Other Investments $ 151.6 $ 125.3
Equity method eligible private equity securities, at fair value    
Debt and Equity Securities, FV-NI [Line Items]    
Other Investments 126.4 83.2
Forward Contracts    
Debt and Equity Securities, FV-NI [Line Items]    
Other Investments 0.0 (3.7)
Other long-term investments    
Debt and Equity Securities, FV-NI [Line Items]    
Other Investments 11.7 4.0
White Mountains    
Debt and Equity Securities, FV-NI [Line Items]    
Other Investments 289.7 208.8
White Mountains | Equity method eligible private equity securities, at fair value    
Debt and Equity Securities, FV-NI [Line Items]    
Other Investments $ 97.3 $ 58.0