Equity incentive plans - Status of Company Stock Options (Details) - Equity Incentive Plan 2011 - Employee stock option - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 30, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Number of shares, Share options outstanding, beginning balance	4,889,585
Number of shares, Granted	1,181,777
Number of Shares, Exercised	(880,346)
Number of shares, Forfeited or expired	(823,489)
Number of shares, Share options outstanding, ending balance	4,367,527	4,889,585
Number of shares, Share options exercisable	2,026,779
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward]
Weighted average exercise price, Share options outstanding, beginning balance	$ 44.82
Weighted average exercise price, Granted	55.04
Weighted average exercise price, Exercised	40.37
Weighted average exercise price, Forfeited or expired	49.38
Weighted average exercise price, Share options outstanding, ending balance	47.63	$ 44.82
Weighted average exercise price, Share options exercisable	$ 44.99
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]
Weighted average remaining contractual term (years), Share options outstanding	4 years 7 months 6 days	5 years 3 months 19 days
Weighted average remaining contractual term (years), Share options exercisable	3 years 10 months 25 days
Aggregate intrinsic value (in millions), Share options outstanding	$ 73.6
Aggregate intrinsic value (in millions), Share options exercisable	$ 39.5