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
	Mar. 31, 2021	Mar. 31, 2020
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 7.7	$ 9.6
Expected return on plan assets	(12.0)	(14.9)
Amortization of prior service credit	(0.1)	(0.1)
Amortization of prior net pension loss	3.0	3.7
Total net periodic pension (benefit) cost	$ (1.4)	$ (1.7)