Retirement Benefits - Schedule of Components of Retirement Benefit Expense (Income) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Interest cost on benefit obligation	$ 8.4	$ 10.7	$ 25.1	$ 31.9
Expected return on assets	(15.3)	(15.2)	(46.0)	(45.4)
Amortization of prior service costs	0.0	0.0	0.1	0.1
Amortization of net losses (gains)	16.0	14.3	47.9	43.0
Retirement benefits expense (income)	9.1	9.8	27.1	29.6
Postretirement Medical and Life Insurance Benefits
Defined Benefit Plan Disclosure [Line Items]
Interest cost on benefit obligation	0.1	0.2	0.4	0.6
Expected return on assets	0.0	0.0	0.0	0.0
Amortization of prior service costs	0.0	0.0	0.0	0.0
Amortization of net losses (gains)	(0.7)	(0.9)	(2.1)	(2.8)
Retirement benefits expense (income)	$ (0.6)	$ (0.7)	$ (1.7)	$ (2.2)