Post-retirement benefits - Components of Net Periodic Benefit Cost (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended
	Sep. 30, 2017	Mar. 31, 2018	Apr. 01, 2017
Defined Benefit Plan Disclosure [Line Items]
—Employer service cost		$ 1.4	$ 1.3
—Interest cost		6.6	8.5
—Expected return on plan assets		(5.8)	(7.0)
—Amortization of net actuarial (gain) loss		(0.2)	0.1
—Settlements	$ (3.9)	0.4	0.0
Net periodic benefit cost		2.4	2.9
Pensions
Defined Benefit Plan Disclosure [Line Items]
—Employer service cost		1.4	1.3
—Interest cost		6.0	7.8
—Expected return on plan assets		(5.8)	(7.0)
—Amortization of net actuarial (gain) loss		0.0	0.1
—Settlements		0.4	0.0
Net periodic benefit cost		2.0	2.2
Other post-retirement benefits
Defined Benefit Plan Disclosure [Line Items]
—Employer service cost		0.0	0.0
—Interest cost		0.6	0.7
—Expected return on plan assets		0.0	0.0
—Amortization of net actuarial (gain) loss		(0.2)	0.0
—Settlements		0.0	0.0
Net periodic benefit cost		$ 0.4	$ 0.7