Retirement Benefit Plans (Components of net periodic benefit cost) (Details) - Retirement Plans - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 2.4	$ 1.1	$ 3.8
Interest cost	9.2	13.4	12.6
Expected return on plan assets	(23.7)	(22.1)	(22.0)
Amortization of prior service cost/(credit)	0.2	0.3	0.2
Amortization of net actuarial (gain)/loss	3.6	2.9	2.1
Settlement	8.0	(0.2)	3.1
Curtailment (gain)/loss	0.0	0.4	0.0
Net periodic benefit cost/(credit)	$ (0.3)	$ (4.2)	$ (0.2)