Post-retirement benefits - Plan asset by asset category (Details) - Pensions - USD ($) $ in Millions	Jan. 01, 2022	Jan. 02, 2021	Dec. 28, 2019
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan asset	$ 829.8	$ 881.4	$ 835.2
Total	829.8	881.4
Quoted prices in active markets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans
Total	7.5	8.4
Significant observable inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans
Total	611.8	633.6
Significant unobservable inputs (Level 3)
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan asset	210.5	239.4	$ 235.9
Total	210.5	239.4
Equity Securities
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan asset	145.3	144.1
Equity Securities | Quoted prices in active markets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan asset	0.0	0.0
Equity Securities | Significant observable inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan asset	145.3	144.1
Equity Securities | Significant unobservable inputs (Level 3)
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan asset	0.0	0.0
—Corporate bonds
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan asset	240.1	190.6
—Corporate bonds | Quoted prices in active markets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan asset	0.0	0.0
—Corporate bonds | Significant observable inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan asset	240.1	190.6
—Corporate bonds | Significant unobservable inputs (Level 3)
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan asset	0.0	0.0
—Government bonds
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan asset	199.6	249.5
—Government bonds | Quoted prices in active markets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan asset	0.0	0.0
—Government bonds | Significant observable inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan asset	199.6	249.5
—Government bonds | Significant unobservable inputs (Level 3)
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan asset	0.0	0.0
Annuities and insurance
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan asset	231.7	288.8
Annuities and insurance | Quoted prices in active markets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan asset	0.0	0.0
Annuities and insurance | Significant observable inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan asset	21.2	49.4
Annuities and insurance | Significant unobservable inputs (Level 3)
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan asset	210.5	239.4
Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan asset	7.5	8.4
Cash and cash equivalents | Quoted prices in active markets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan asset	7.5	8.4
Cash and cash equivalents | Significant observable inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan asset	0.0	0.0
Cash and cash equivalents | Significant unobservable inputs (Level 3)
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan asset	0.0	0.0
Other
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan asset	5.6	0.0
Other | Quoted prices in active markets (Level 1)
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan asset	0.0	0.0
Other | Significant observable inputs (Level 2)
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan asset	5.6	0.0
Other | Significant unobservable inputs (Level 3)
Defined Benefit Plans and Other Postretirement Benefit Plans
Plan asset	$ 0.0	$ 0.0