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Employee Benefits - Schedule of Components of Our Company-Sponsored Pension Plan Costs - (Details) - Qualified and Nonqualified - Defined Benefit Pension Plans - USD ($)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Defined Benefit Plan Disclosure [Line Items]        
Interest cost $ 9.6 $ 11.4 $ 19.2 $ 22.8
Expected return on plan assets (14.9) (14.3) (29.9) (28.6)
Amortization of prior service credit (0.1) (0.1) (0.2) (0.2)
Amortization of prior net pension loss 3.7 3.2 7.5 6.4
Total net periodic pension cost $ (1.7) $ 0.2 $ (3.4) $ 0.4