Commitments and contingencies Details (Details) $ in Thousands	3 Months Ended
	Mar. 30, 2019 USD ($)	Mar. 31, 2018	Apr. 01, 2017
Loss contingency, number of claims alleged	3
Credit Concentration Risk [Member]
Concentration risk, percentage		12.00%	11.00%
Customer Concentration Risk [Member]
Pre-petition accounts receivable	$ 14,000
Former Davey employee defendants [Member]
Loss contingency, number of defendants	1
Wolf employee defendants [Member]
Loss contingency, number of defendants	2
Former Wolf employee defendants [Member]
Loss contingency, number of defendants	1
Employee and former employee defendants [Member]
Loss contingency, number of defendants	4
Former Wolf employee defendants indicted [Member]
Loss contingency, number of defendants	2
Other individuals [Member]
Loss contingency, number of defendants	1