Post Retirement Benefit Plans - Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Components of net periodic benefit costs:
Service cost	$ 6.4	$ 7.5	$ 7.8
Interest cost	2.6	2.0	1.7
Expected return on plan assets	(2.0)	(1.9)	(1.7)
Amortization of net loss	2.0	3.8	4.8
Net periodic benefit costs	$ 9.0	$ 11.4	$ 12.6