Post-retirement benefits - Funded status (Details) - USD ($) $ in Millions	Dec. 28, 2019	Dec. 29, 2018
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position
Post-retirement benefit obligations	$ 151.2	$ 155.9
Pensions
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position
Pension surplus	(38.1)	(52.6)
Accrued expenses and other current liabilities	2.3	2.4
Post-retirement benefit obligations	98.7	102.4
Net unfunded pension obligation	62.9	52.2
Defined Benefit Plan, Pension Plan with Project Benefit Obligation in Excess of Plan Assets [Abstract]
—Aggregate projected benefit obligation	391.7	378.2
—Aggregate plan assets	290.7	273.4
Plans whose accumulated benefit obligation was in excess of the plan assets:
—Aggregate accumulated benefit obligation	381.6	372.3
—Aggregate plan assets	286.2	273.0
Other post-retirement benefits
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position
Accrued expenses and other current liabilities	5.4	6.5
Post-retirement benefit obligations	52.5	53.5
Net unfunded pension obligation	$ 57.9	$ 60.0