Share-based compensation - Stock Option and SAR Rollforward (Details) - $ / shares	12 Months Ended
	Dec. 28, 2019	Dec. 29, 2018
Number of options
Beginning balance (shares)	20,032,966
Granted (shares)	1,967,115
Forfeited (shares)	(1,504,986)
Expired (shares)	(2,828)
Exercised (shares)	(270,997)	(79,014)
Ending balance (shares)	20,221,270	20,032,966
Weighted average exercise price $
Beginning balance (usd per share)	$ 8.16
Granted (usd per share)	17.49
Forfeited (usd per share)	8.14
Expired (usd per share)	16.28
Exercised (usd per share)	6.61
Ending balance (usd per share)	$ 9.09	$ 8.16
—Share options
Number of options
Beginning balance (shares)	582,717
Granted (shares)	1,099,505
Forfeited (shares)	(69,862)
Expired (shares)	(1,875)
Ending balance (shares)	1,610,485	582,717
Weighted average exercise price $
Beginning balance (usd per share)	$ 17.14
Granted (usd per share)	16.46
Forfeited (usd per share)	16.86
Expired (usd per share)	17.72
Ending balance (usd per share)	$ 16.69	$ 17.14
Exercisable at the end of the period (shares)	2,864,411
Exercisable at the end of the period (usd per share)	$ 7.37
Vested and expected to vest at the end of the period (shares)	6,978,259
Vested and expected to vest at the end of the period (usd per share)	$ 10.81
—Share options | —Tier I
Number of options
Beginning balance (shares)	4,212,537
Forfeited (shares)	(115,685)
Exercised (shares)	(270,997)
Ending balance (shares)	3,825,855	4,212,537
Weighted average exercise price $
Beginning balance (usd per share)	$ 7.03
Forfeited (usd per share)	6.61
Exercised (usd per share)	6.61
Ending balance (usd per share)	$ 7.07	$ 7.03
—Share options | —Tier II
Number of options
Beginning balance (shares)	4,837,780
Forfeited (shares)	(432,440)
Ending balance (shares)	4,405,340	4,837,780
Weighted average exercise price $
Beginning balance (usd per share)	$ 6.97
Forfeited (usd per share)	6.58
Ending balance (usd per share)	$ 7.01	$ 6.97
—Share options | —Tier III
Number of options
Beginning balance (shares)	4,837,780
Forfeited (shares)	(432,440)
Ending balance (shares)	4,405,340	4,837,780
Weighted average exercise price $
Beginning balance (usd per share)	$ 6.97
Forfeited (usd per share)	6.58
Ending balance (usd per share)	$ 7.01	$ 6.97
—Share options | —Tier IV
Number of options
Beginning balance (shares)	4,837,780
Forfeited (shares)	(432,440)
Ending balance (shares)	4,405,340	4,837,780
Weighted average exercise price $
Beginning balance (usd per share)	$ 10.46
Forfeited (usd per share)	9.88
Ending balance (usd per share)	$ 10.52	$ 10.46
—SARs
Number of options
Beginning balance (shares)	724,372
Granted (shares)	71,150
Forfeited (shares)	(22,119)
Expired (shares)	(953)
Ending balance (shares)	772,450	724,372
Weighted average exercise price $
Beginning balance (usd per share)	$ 8.17
Granted (usd per share)	16.46
Forfeited (usd per share)	15.43
Expired (usd per share)	13.44
Ending balance (usd per share)	$ 8.72	$ 8.17
—Premium-priced options
Number of options
Granted (shares)	796,460
Ending balance (shares)	796,460
Weighted average exercise price $
Granted (usd per share)	$ 19.00
Ending balance (usd per share)	$ 19.00