Commitments and Contingencies (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Loss contingency number of claims alleged	3
Letter of credit [Member]
Letters of credit outstanding	$ 72,565	$ 63,242
Former Davey 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 [Member]
Loss contingency, number of defendants	1
Former Wolf employee defendants indicted [Member]
Loss contingency, number of defendants	2
Other individuals [Member]
Loss contingency, number of defendants	1