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Employee and Agent Benefits - Net Periodic Benefit Cost (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2024
Dec. 31, 2023
Dec. 31, 2022
Plan actuarial gain and loss amortization disclosures      
Allowable corridor not used for amortization of actuarial gains and losses of qualified pension plan (as a percent) 10.00%    
Pension benefits      
Components of Net Periodic Benefit Cost (Income)      
Service cost $ 59.2 $ 55.6 $ 78.4
Interest cost $ 161.5 $ 160.1 $ 112.8
Interest cost, location on consolidated statements of operations Operating expenses Operating expenses Operating expenses
Expected return on plan assets $ (170.4) $ (163.2) $ (178.9)
Expected return on plan assets, location on consolidated statements of operations Operating expenses Operating expenses Operating expenses
Amortization of prior service (benefit) cost $ (16.7) $ (16.8) $ (16.8)
Amortization of prior service (benefit) cost, location on consolidated statements of operations Operating expenses Operating expenses Operating expenses
Recognized net actuarial (gain) loss $ 39.0 $ 41.0 $ 55.7
Recognized net actuarial (gain) loss, location on consolidated statements of operations Operating expenses Operating expenses Operating expenses
Settlement $ 0.2    
Settlement, location on consolidated statements of operations Operating expenses    
Net periodic benefit cost (income) $ 72.8 $ 76.7 $ 51.2
Other changes recognized in accumulated other comprehensive (income) loss      
Net actuarial (gain) loss (14.5) 24.4  
Amortization of gain (loss) (39.0) (41.0)  
Amortization of prior service benefit (cost) 16.7 16.8  
Settlement (0.2)    
Total recognized in pre-tax accumulated other comprehensive (income) loss (37.0) 0.2  
Total recognized in net periodic benefit cost and pre-tax accumulated other comprehensive (income) loss 35.8 76.9  
Other postretirement benefits      
Components of Net Periodic Benefit Cost (Income)      
Interest cost $ 3.2 $ 3.4 $ 2.3
Interest cost, location on consolidated statements of operations Operating expenses Operating expenses Operating expenses
Expected return on plan assets $ (4.5) $ (4.6) $ (4.9)
Expected return on plan assets, location on consolidated statements of operations Operating expenses Operating expenses Operating expenses
Amortization of prior service (benefit) cost $ (1.1) $ (0.8) $ (1.1)
Amortization of prior service (benefit) cost, location on consolidated statements of operations Operating expenses Operating expenses Operating expenses
Recognized net actuarial (gain) loss $ (0.9) $ (1.0) $ (1.1)
Recognized net actuarial (gain) loss, location on consolidated statements of operations Operating expenses Operating expenses Operating expenses
Net periodic benefit cost (income) $ (3.3) $ (3.0) $ (4.8)
Other changes recognized in accumulated other comprehensive (income) loss      
Net actuarial (gain) loss (6.4) (5.2)  
Amortization of gain (loss) 0.9 1.0  
Amortization of prior service benefit (cost) 1.1 0.8  
Total recognized in pre-tax accumulated other comprehensive (income) loss (4.4) (3.4)  
Total recognized in net periodic benefit cost and pre-tax accumulated other comprehensive (income) loss $ (7.7) $ (6.4)