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Employee and Agent Benefits - Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Components of Net Periodic Benefit Cost (Income)      
Contributions made by employer to fund qualified and nonqualified pension plans $ 0.4   $ 1.5
Pension benefits      
Components of Net Periodic Benefit Cost (Income)      
Service cost 20.8 $ 18.1  
Interest cost $ 25.8 $ 29.3  
Interest cost, location on consolidated statements of operations Operating expenses Operating expenses  
Expected return on plan assets $ (45.7) $ (39.2)  
Expected return on plan assets, location on consolidated statements of operations Operating expenses Operating expenses  
Amortization of prior service (benefit) cost $ (4.2) $ (4.2)  
Amortization of prior service (benefit) cost, location on consolidated statements of operations Operating expenses Operating expenses  
Recognized net actuarial (gain) loss $ 17.6 $ 18.8  
Recognized net actuarial (gain) loss, location on consolidated statements of operations Operating expenses Operating expenses  
Net periodic benefit cost (income) $ 14.3 $ 22.8  
Contributions made by employer to fund qualified and nonqualified pension plans 35.7    
Pension benefits | Maximum      
Components of Net Periodic Benefit Cost (Income)      
Amount of possible contributions to be made during the current fiscal year to the qualified and nonqualified pension plans combined 125.0    
Other postretirement benefits      
Components of Net Periodic Benefit Cost (Income)      
Interest cost $ 0.5 $ 0.7  
Interest cost, location on consolidated statements of operations Operating expenses Operating expenses  
Expected return on plan assets $ (1.1) $ (9.0)  
Expected return on plan assets, location on consolidated statements of operations Operating expenses Operating expenses  
Amortization of prior service (benefit) cost $ (0.3) $ (0.2)  
Amortization of prior service (benefit) cost, location on consolidated statements of operations Operating expenses Operating expenses  
Recognized net actuarial (gain) loss $ (0.1)    
Recognized net actuarial (gain) loss, location on consolidated statements of operations Operating expenses Operating expenses  
Net periodic benefit cost (income) $ (1.0) $ (8.5)