Other Postretirement Benefit Plans - Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	3 Months Ended					12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Recognition of net actuarial (gains) losses	$ 21.6	$ 11.9	$ 8.8	$ 21.1	$ 16.9	$ 18.5	$ (4.1)	$ 12.8
Accrued postretirement liability
Defined Benefit Plan Disclosure [Line Items]
Service cost						0.2	0.2	0.2
Interest cost						2.1	5.9	7.6
Expected return on plan assets						(0.4)	(3.2)	(3.7)
Amortization of prior service credit						(9.8)	(5.4)	(1.7)
Recognition of net actuarial (gains) losses						1.4	(18.0)	(16.7)
Net periodic benefit (credit) cost						$ (6.5)	$ (20.5)	$ (14.3)