Employee Benefits - Schedule of Costs of Retirement Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net periodic benefit cost:
Interest cost	$ 37.1	$ 45.7
Amortization of prior net service credit	(14.3)	(12.8)
Other changes in plan assets and benefit obligations recognized in other comprehensive loss (income):
Net actuarial (gains) losses and other adjustments	(204.6)	51.7	$ (1.0)
Settlement adjustment	(3.6)	(1.8)	(10.9)
Amortization of prior net losses	(12.7)	(12.9)	(14.6)
Other Postretirement Benefits Plan
Net periodic benefit cost:
Interest cost	37.1	45.7	44.1
Expected return on plan assets	(61.1)	(57.3)	(60.0)
Amortization of prior net losses	14.3	12.8	14.6
Amortization of prior net service credit	(0.4)	(0.4)	(0.4)
Settlement adjustment	3.6	1.8	10.9
Net periodic pension cost (benefit)	(6.5)	2.6	9.2
Other changes in plan assets and benefit obligations recognized in other comprehensive loss (income):
Net actuarial (gains) losses and other adjustments	(216.7)	51.9	(0.9)
Settlement adjustment	(3.6)	(1.8)	(10.9)
Amortization of prior net losses	(14.3)	(12.8)	(14.6)
Amortization of prior net service credit	0.4	0.4	0.4
Total recognized in other comprehensive loss (income)	(234.2)	37.7	(26.0)
Total recognized in net periodic benefit cost and other comprehensive loss (income)	$ (240.7)	$ 40.3	$ (16.8)