Post Retirement Benefit Plans - Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Components of net periodic benefit costs:
Service cost	$ 8.2	$ 6.4	$ 7.5
Interest cost	1.1	2.6	2.0
Expected return on plan assets	(2.5)	(2.0)	(1.9)
Amortization of net loss	4.7	2.0	3.8
Net periodic benefit costs	$ 11.5	$ 9.0	$ 11.4