Post Retirement Benefit Plans - Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Components of net periodic benefit costs:
Service cost	$ 7.5	$ 7.8	$ 6.8
Interest cost	2.0	1.7	2.2
Expected return on plan assets	(1.9)	(1.7)	(1.8)
Amortization of net loss	3.8	4.8	4.1
Net periodic benefit costs	$ 11.4	$ 12.6	$ 11.3