Retirement Benefits - Schedule of Components of Retirement Benefit Expense (Income) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Interest cost on benefit obligation	$ 8.9	$ 8.4
Expected return on assets	(15.7)	(15.3)
Amortization of net losses (gains)	7.7	15.9
Retirement benefits expense (income)	0.9	9.0
Postretirement Medical and Life Insurance Benefits
Defined Benefit Plan Disclosure [Line Items]
Interest cost on benefit obligation	0.1	0.1
Expected return on assets	0.0	0.0
Amortization of net losses (gains)	(0.7)	(0.6)
Retirement benefits expense (income)	$ (0.6)	$ (0.5)