Commitments and contingencies Details (Details)	9 Months Ended
	Sep. 29, 2018
Loss contingency, number of claims alleged	3
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
Defendants for which Davey has retained separate counsel [Member]
Loss contingency, number of defendants	3