Post Retirement Benefit Plans - Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of net periodic benefit costs:
Service cost	$ 7.8	$ 6.8	$ 7.2
Interest cost	1.7	2.2	2.5
Expected return on plan assets	(1.7)	(1.8)	(2.3)
Settlement loss recognized			10.2
Amortization of net loss	4.8	4.1	4.1
Net periodic benefit costs	$ 12.6	$ 11.3	$ 21.7