Share-based compensation - Stock Option and SAR Rollforward (Details) - $ / shares	12 Months Ended
	Jan. 02, 2021	Dec. 28, 2019
Number of options
Beginning balance (shares)	20,221,270
Granted (shares)	1,274,730
Forfeited (shares)	(2,215,243)
Expired (shares)	(104,647)
Exercised (shares)	(468,890)	(270,997)
Ending balance (shares)	18,707,220	20,221,270
Weighted average exercise price $
Beginning balance (usd per share)	$ 9.09
Granted (usd per share)	12.51
Forfeited (usd per share)	9.70
Expired (usd per share)	15.68
Exercised (usd per share)	6.65
Ending balance (usd per share)	$ 9.28	$ 9.09
—Share options
Number of options
Beginning balance (shares)	1,610,485
Granted (shares)	1,205,369
Forfeited (shares)	(188,102)
Expired (shares)	(62,686)
Ending balance (shares)	2,565,066	1,610,485
Weighted average exercise price $
Beginning balance (usd per share)	$ 16.69
Granted (usd per share)	12.54
Forfeited (usd per share)	16.31
Expired (usd per share)	17.19
Ending balance (usd per share)	$ 14.75	$ 16.69
Exercisable at the end of the period (shares)	3,394,050
Exercisable at the end of the period (usd per share)	$ 8.47
Vested and expected to vest at the end of the period (shares)	7,343,504
Vested and expected to vest at the end of the period (usd per share)	$ 11.19
—Share options | —Tier I
Number of options
Beginning balance (shares)	3,825,855
Forfeited (shares)	(149,522)
Expired (shares)	(41,961)
Exercised (shares)	(468,890)
Ending balance (shares)	3,165,482	3,825,855
Weighted average exercise price $
Beginning balance (usd per share)	$ 7.07
Forfeited (usd per share)	9.56
Expired (usd per share)	13.44
Exercised (usd per share)	6.65
Ending balance (usd per share)	$ 6.93	$ 7.07
—Share options | —Tier II
Number of options
Beginning balance (shares)	4,405,340
Forfeited (shares)	(625,873)
Ending balance (shares)	3,779,467	4,405,340
Weighted average exercise price $
Beginning balance (usd per share)	$ 7.01
Forfeited (usd per share)	7.76
Ending balance (usd per share)	$ 6.89	$ 7.01
—Share options | —Tier III
Number of options
Beginning balance (shares)	4,405,340
Forfeited (shares)	(625,873)
Ending balance (shares)	3,779,467	4,405,340
Weighted average exercise price $
Beginning balance (usd per share)	$ 7.01
Forfeited (usd per share)	7.76
Ending balance (usd per share)	$ 6.89	$ 7.01
—Share options | —Tier IV
Number of options
Beginning balance (shares)	4,405,340
Forfeited (shares)	(625,873)
Ending balance (shares)	3,779,467	4,405,340
Weighted average exercise price $
Beginning balance (usd per share)	$ 10.52
Forfeited (usd per share)	11.63
Ending balance (usd per share)	$ 10.34	$ 10.52
—SARs
Number of options
Beginning balance (shares)	772,450
Granted (shares)	69,361
Ending balance (shares)	841,811	772,450
Weighted average exercise price $
Beginning balance (usd per share)	$ 8.72
Granted (usd per share)	12.08
Ending balance (usd per share)	$ 9.00	$ 8.72
—Premium-priced options
Number of options
Beginning balance (shares)	796,460
Ending balance (shares)	796,460	796,460
Weighted average exercise price $
Beginning balance (usd per share)	$ 19.00
Ending balance (usd per share)	$ 19.00	$ 19.00