Share-based compensation - Stock Option and SAR Rollforward (Details) - $ / shares	6 Months Ended
	Jun. 27, 2020	Jun. 29, 2019
Number of options
Beginning balance (shares)	20,221,270
Granted (shares)	1,262,475
Forfeited (shares)	(1,831,974)
Expired (shares)	(3,055)
Exercised (shares)	(400,573)	(246,924)
Ending balance (shares)	19,248,143
Weighted average exercise price $
Beginning balance (usd per share)	$ 9.09
Granted (usd per share)	12.52
Forfeited (usd per share)	10.01
Expired (usd per share)	16.46
Exercised (usd per share)	6.64
Ending balance (usd per share)	$ 9.28
—Share options
Number of options
Beginning balance (shares)	1,610,485
Granted (shares)	1,193,114
Forfeited (shares)	(154,408)
Expired (shares)	(3,055)
Ending balance (shares)	2,646,136
Weighted average exercise price $
Beginning balance (usd per share)	$ 16.69
Granted (usd per share)	12.55
Forfeited (usd per share)	16.79
Expired (usd per share)	16.46
Ending balance (usd per share)	$ 14.82
Exercisable at the end of the period (shares)	3,376,495
Exercisable at the end of the period (usd per share)	$ 8.65
Vested and expected to vest at the end of the period (shares)	7,513,195
Vested and expected to vest at the end of the period (usd per share)	$ 11.22
—Share options | —Tier I
Number of options
Beginning balance (shares)	3,825,855
Forfeited (shares)	(149,522)
Exercised (shares)	(400,573)
Ending balance (shares)	3,275,760
Weighted average exercise price $
Beginning balance (usd per share)	$ 7.07
Forfeited (usd per share)	9.56
Exercised (usd per share)	6.64
Ending balance (usd per share)	$ 7.01
—Share options | —Tier II
Number of options
Beginning balance (shares)	4,405,340
Forfeited (shares)	(509,348)
Ending balance (shares)	3,895,992
Weighted average exercise price $
Beginning balance (usd per share)	$ 7.01
Forfeited (usd per share)	8.03
Ending balance (usd per share)	$ 6.88
—Share options | —Tier III
Number of options
Beginning balance (shares)	4,405,340
Forfeited (shares)	(509,348)
Ending balance (shares)	3,895,992
Weighted average exercise price $
Beginning balance (usd per share)	$ 7.01
Forfeited (usd per share)	8.03
Ending balance (usd per share)	$ 6.88
—Share options | —Tier IV
Number of options
Beginning balance (shares)	4,405,340
Forfeited (shares)	(509,348)
Ending balance (shares)	3,895,992
Weighted average exercise price $
Beginning balance (usd per share)	$ 10.52
Forfeited (usd per share)	12.04
Ending balance (usd per share)	$ 10.32
—SARs
Number of options
Beginning balance (shares)	772,450
Granted (shares)	69,361
Ending balance (shares)	841,811
Weighted average exercise price $
Beginning balance (usd per share)	$ 8.72
Granted (usd per share)	12.08
Ending balance (usd per share)	$ 9.00
—Premium-priced options
Number of options
Beginning balance (shares)	796,460
Ending balance (shares)	796,460
Weighted average exercise price $
Beginning balance (usd per share)	$ 19.00
Ending balance (usd per share)	$ 19.00