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Investments (Narrative) (Details)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2017
USD ($)
security
state
Dec. 31, 2016
USD ($)
security
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 $ 233.9 $ 215.3
Percentage of revenue securities 44.00% 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 | security 603 796
Percentage of residential mortgage-backed holdings exposure to sub-prime mortgage collateral 34.00%  
Minimum    
Investment [Line Items]    
Outstanding balance of commercial mortgage loans $ 0.1 $ 0.1
Maximum    
Investment [Line Items]    
Outstanding balance of commercial mortgage loans 12.5 12.6
Energy Sector | Foreign corporate    
Investment [Line Items]    
Investment in securities 659.5 641.9
Unrealized gain (loss) on investments 57.9 51.6
Europe | Corporate Fixed Maturity and Equity Securities    
Investment [Line Items]    
Investment in securities 700.1 693.3
Unrealized gain (loss) on investments $ 61.3 $ 54.2
Investments | Internal Investment Grade | Energy Sector | Foreign corporate    
Investment [Line Items]    
Percentage of investments held 84.00% 84.00%
Geographic Concentration Risk | Investments | Canada | Foreign Government Fixed Maturity Securities    
Investment [Line Items]    
Percentage of investments held 77.00% 78.00%
Geographic Concentration Risk | Investments | Brazil | Foreign Government Fixed Maturity Securities    
Investment [Line Items]    
Percentage of investments held 14.00% 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 2.00% 3.00%
Geographic Concentration Risk | Investments | Unspecified European Country | Corporate Fixed Maturity and Equity Securities    
Investment [Line Items]    
Percentage of investments held 3.00% 4.00%
Investment Sector Concentration Risk | Investments | Europe | Financial Services Sector | Corporate Fixed Maturity and Equity Securities    
Investment [Line Items]    
Percentage of investments held 23.00% 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 7.00%