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Investments (Narrative) (Details)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2017
USD ($)
investment
state
Sep. 30, 2016
Dec. 31, 2016
USD ($)
investment
state
Investment [Line Items]      
Number of individual states exceeding overall investment portfolio exposure | state 0   0
Maximum individual state exposure 0.50%   0.50%
Advance refunded or escrowed-to-maturity securities $ 186.7   $ 215.3
Percentage of revenue securities 49.00%   46.00%
Percentage of securities representing gross unrealized losses 2.00%   2.00%
Percentage of gross unrealized losses in a continuous loss position less than twelve months 90.00%   89.00%
Individual securities comprising total gross unrealized losses | investment 593   796
Percentage of residential mortgage-backed holdings exposure to sub-prime mortgage collateral 37.00%    
Minimum      
Investment [Line Items]      
Outstanding balance of commercial mortgage loans (less than $1.0) $ 0.1   $ 0.1
Maximum      
Investment [Line Items]      
Outstanding balance of commercial mortgage loans (less than $1.0) 12.8   12.6
Energy Sector | Foreign corporate      
Investment [Line Items]      
Investment in securities 676.3   641.9
Unrealized gain (loss) on investments 64.1   51.6
Europe | Corporate Fixed Maturity and Equity Securities      
Investment [Line Items]      
Investment in securities 627.5   693.3
Unrealized gain (loss) on investments $ 64.2   $ 54.2
Investments | Energy Sector | Foreign corporate | Internal Investment Grade      
Investment [Line Items]      
Percentage of investments held 87.00%   84.00%
Geographic Concentration Risk | Investments | Canada | Foreign Government Fixed Maturity Securities      
Investment [Line Items]      
Percentage of investments held 78.00%   78.00%
Geographic Concentration Risk | Investments | Brazil | Foreign Government Fixed Maturity Securities      
Investment [Line Items]      
Percentage of investments held 13.00%   11.00%
Geographic Concentration Risk | Investments | Germany | Foreign Government Fixed Maturity Securities      
Investment [Line Items]      
Percentage of investments held 4.00%   4.00%
Geographic Concentration Risk | Investments | Other Countries (less than 3%) | Foreign Government Fixed Maturity Securities      
Investment [Line Items]      
Percentage of investments held 3.00%   3.00%
Geographic Concentration Risk | Investments | United Kingdom | Corporate Fixed Maturity and Equity Securities      
Investment [Line Items]      
Percentage of investments held 4.00%   4.00%
Investment Sector Concentration Risk | Investments | Europe | Financial Services Sector | Corporate Fixed Maturity and Equity Securities      
Investment [Line Items]      
Percentage of investments held 28.00% 23.00%  
Investment Hedging Concentration Risk | Investments | Europe | Pound and Euro Denominated | Not Hedged to U.S. Dollars | Corporate Fixed Maturity and Equity Securities      
Investment [Line Items]      
Percentage of investments held 8.00%