Acquisition - Summary of acquisition by acquisition (Details) - USD ($) $ in Thousands		6 Months Ended
	Jul. 02, 2018	Jun. 30, 2019	Dec. 31, 2018
Goodwill		$ 56,596	$ 56,588
Huen Electric Inc [Member]
Consideration paid	$ 47,082	47,082
Preliminary estimated net asset adjustments	10,749	10,834
Total consideration, net of net asset adjustments	57,831	57,916
Accounts receivable, net	33,903	33,696
Contract assets	10,570	11,580
Other current and long term assets	88	77
Property and equipment	3,188	3,188
Intangible assets	0	24,300
Accounts payable	(9,592)	(10,866)
Contract liabilities	(6,394)	(5,869)
Other current liabilities	(6,570)	(6,521)
Net identifiable assets and liabilities	25,193	49,585
Unallocated intangible assets	9,800	0
Total acquired assets and liabilities	34,993	49,585
Fair value of acquired noncontrolling interest	(1,273)	(1,280)
Goodwill	$ 24,111	9,611
Consideration paid		0
Preliminary estimated net asset adjustments		85
Total consideration, net of net asset adjustments		85
Accounts receivable, net		(207)
Contract assets		1,010
Other current and long term assets		(11)
Property and equipment		0
Intangible assets		24,300
Accounts payable		(1,274)
Contract liabilities		525
Other current liabilities		49
Net identifiable assets and liabilities		24,392
Unallocated intangible assets		(9,800)
Total acquired assets and liabilities		14,592
Fair value of acquired noncontrolling interest		(7)
Goodwill		$ (14,500)