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Investments (Narrative) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Other private securities      
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]      
Realized gain on disposal $ 21.7    
Unrealized gain on change in method of accounting $ 8.1    
Other invested assets      
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]      
Income (loss) from equity method investments   $ (6.3) $ (9.3)
Minimum | Investments with redemption restrictions      
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]      
Redemption restriction, lock-up period 1 year    
Minimum | Investments without redemption restrictions | Hedge funds      
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]      
Redemption notice period 30 days    
Gate trigger, percentage 15.00%    
Maximum | Investments with redemption restrictions      
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]      
Redemption restriction, lock-up period 3 years    
Maximum | Investments without redemption restrictions | Hedge funds      
Fair Value, Investments, Entities that Calculate Net Asset Value Per Share [Line Items]      
Gate trigger, percentage 25.00%