Note 16 - Retirement Benefit Plans - Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Service cost	$ 0.0	$ 0.0	$ (0.6)
Settlement loss	0.0	0.0	(1.1)
Curtailment loss	0.0	0.0	(0.3)
Interest cost	(27.6)	(16.9)	(13.5)
Amortization of actuarial loss	(0.5)	0.0	0.0
Amortization of prior service cost	(0.4)	(0.4)	(0.4)
Expected return on plan assets	28.7	13.8	11.3
Net periodic benefit credit (cost)	0.2	(3.5)	(4.6)
Contributions to money purchase section	(11.6)	(11.5)	(11.3)
Total cost	(11.4)	(15.0)	$ (15.9)
Pension Plan [Member]
Interest cost	(27.6)	(16.9)
Amortization of actuarial loss	$ (0.5)	$ (0.0)
Discount rate	4.80%	1.90%	1.30%
Inflation — salaries			2.50%
Inflation — RPI	3.30%	3.40%	2.90%
Inflation — CPI	2.70%	2.80%	2.20%
Pension increases (RPI capped at 5% p.a.)	3.20%	3.30%	2.90%
Pension increases (RPI capped at 2.5% p.a.)	2.10%	2.20%	2.10%
Expected return on plan assets	4.40%	1.60%	1.20%
Amortization period for net actuarial gains at beginning of the year (Year)	28 years	29 years	9 years