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, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 7.7	$ 9.6	$ 15.3	$ 19.2
Expected return on plan assets	(12.0)	(14.9)	(24.0)	(29.9)
Amortization of prior service credit	(0.1)	(0.1)	(0.2)	(0.2)
Amortization of prior net pension loss	3.0	3.7	6.1	7.5
Settlement adjustment	0.3		0.3
Total net periodic pension (benefit) cost	$ (1.1)	$ (1.7)	$ (2.5)	$ (3.4)