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Employee and Agent Benefits - Pension Plan Changes and Plan Gains/Losses (Details) - Pension benefits - USD ($)
$ in Millions
12 Months Ended
May 31, 2019
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Components of Net Periodic Benefit Cost (Income)        
Actuarial gain (loss) $ (59.0) $ (506.4) $ 220.9  
Total plan amendment gain 122.4      
Increase in benefit obligation 237.6      
Asset gain on plan assets $ 178.6 520.9 (122.2)  
Prior service cost (benefit) related to current year amendments       $ (23.1)
Contributions        
Contributions made by employer to fund qualified and nonqualified pension plans   $ 30.5 187.8  
Nonqualified benefit plans        
Components of Net Periodic Benefit Cost (Income)        
Prior service cost (benefit) related to current year amendments     $ (6.4)