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Investments - Cost or Amortized Cost and Fair Value of Available-for-Sale Securities (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]    
Document Period End Date Dec. 31, 2018  
Cost or amortized cost   $ 2,517.5
Gross unrealized holding gains   126.9
Gross unrealized holding losses   (30.3)
Fair value   2,614.1
Fixed maturities:    
Debt Securities, Available-for-sale [Line Items]    
Cost or amortized cost   2,173.1
Gross unrealized holding gains $ 14.4 40.0
Gross unrealized holding losses (43.1) (20.3)
Fair value   2,192.8
Equity securities:    
Debt Securities, Available-for-sale [Line Items]    
Cost or amortized cost   318.6
Gross unrealized holding gains   56.7
Gross unrealized holding losses   (10.0)
Fair value 315.0 365.3
Other Invested Assets [Member]    
Debt Securities, Available-for-sale [Line Items]    
Cost or amortized cost   25.8
Gross unrealized holding gains   30.2
Gross unrealized holding losses   0.0
Fair value   56.0
U.S. treasury securities and obligations of U.S. government agencies | Fixed maturities:    
Debt Securities, Available-for-sale [Line Items]    
Cost or amortized cost   433.8
Gross unrealized holding gains 3.2 9.3
Gross unrealized holding losses (9.2) (6.2)
Fair value   436.9
Obligations of states and political subdivisions | Fixed maturities:    
Debt Securities, Available-for-sale [Line Items]    
Cost or amortized cost   507.1
Gross unrealized holding gains 7.3 19.1
Gross unrealized holding losses (0.9) (0.4)
Fair value   525.8
Corporate securities | Fixed maturities:    
Debt Securities, Available-for-sale [Line Items]    
Cost or amortized cost   527.5
Gross unrealized holding gains 0.6 4.5
Gross unrealized holding losses (11.7) (2.3)
Fair value   529.7
U.S. government agencies mortgage-backed securities | Fixed maturities:    
Debt Securities, Available-for-sale [Line Items]    
Cost or amortized cost   704.7
Gross unrealized holding gains 3.3 7.1
Gross unrealized holding losses (21.3) (11.4)
Fair value   700.4
Large-cap securities | Equity securities:    
Debt Securities, Available-for-sale [Line Items]    
Cost or amortized cost   62.4
Gross unrealized holding gains   35.1
Gross unrealized holding losses   (0.7)
Fair value   96.8
Small-cap securities | Equity securities:    
Debt Securities, Available-for-sale [Line Items]    
Gross unrealized holding losses   (9.3)
Mutual and exchange traded funds [Domain] [Member] | Equity securities:    
Debt Securities, Available-for-sale [Line Items]    
Cost or amortized cost   256.2
Gross unrealized holding gains   21.6
Fair value $ 237.8 $ 268.5