Shareholders' equity - Reconciliation of basic to diluted weighted average common shares outstanding (Details) - shares shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 28, 2020	Jun. 30, 2019	Jun. 28, 2020	Jun. 30, 2019
Equity [Abstract]
Basic (in shares)	46,442	46,172	46,412	46,111
Dilutive effect of share-based awards (in shares)	800	864	825	878
Diluted (in shares)	47,242	47,036	47,237	46,989