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Net Investment Income (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Schedule of Investment Income, Reported Amounts, by Category [Line Items]        
Investment income $ 19.3 $ 18.7 $ 58.0 $ 56.4
Investment expenses 0.8 0.8 2.6 2.3
Net investment income 18.5 17.9 55.4 54.1
Debt Securities [Member]        
Schedule of Investment Income, Reported Amounts, by Category [Line Items]        
Investment income related to fixed maturity securities and short-term investments and cash equivalents 17.4 16.9 52.4 50.8
Equity Securities [Member]        
Schedule of Investment Income, Reported Amounts, by Category [Line Items]        
Investment income related to equity securities 1.7 1.7 5.2 5.4
Short-term Investments [Member]        
Schedule of Investment Income, Reported Amounts, by Category [Line Items]        
Investment income related to fixed maturity securities and short-term investments and cash equivalents $ 0.2 $ 0.1 $ 0.4 $ 0.2