Retirement Benefits - Savings Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Contribution Plan Disclosure [Line Items]
Matching defined contribution expense	$ 11.9	$ 11.4	$ 9.9
U.S. 401(K)
Defined Contribution Plan Disclosure [Line Items]
Matching defined contribution expense	2.0	6.5	5.0
Other plans
Defined Contribution Plan Disclosure [Line Items]
Matching defined contribution expense	$ 9.9	$ 4.9	$ 4.9