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Investments (Narrative) (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
USD ($)
security
Dec. 31, 2016
USD ($)
security
Dec. 31, 2015
USD ($)
Investments [Abstract]      
Maximum restricted level of investment upon stockholders equity 10.00% 10.00%  
Gross realized gains on investments $ 8.0 $ 2.1 $ 6.7
Gross realized losses on investments 1.2 0.8 0.9
Proceeds from sale of available-for-sale securities $ 29.1 $ 28.5 $ 51.7
Number of debt security positions | security 25 28  
Recognized other-than-temporary losses on our debt securities portfolio $ 5.9    
Securities pledged for the benefit of various state insurance departments and reinsurers 26.2 $ 21.1  
Fair value of impaired debt securities $ 4.4