Commitments and Contingencies - Schedule of Restructuring Activities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restructuring Cost and Reserve [Line Items]
Restructuring reserve	$ 492	$ 201	$ 81
Restructuring expenses	1,577	1,438	824
Payments for restructuring	(1,286)	(1,318)
Employee Severance [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring expenses	1,516
Facility Closing [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring expenses	61
UNITED STATES | Operating Segments [Member] | United States Electrical Construction And Facilities Services
Restructuring Cost and Reserve [Line Items]
Restructuring reserve	452	0	0
Reversal of restructuring expenses		0
Restructuring expenses	452
Payments for restructuring	0	0
UNITED STATES | Operating Segments [Member] | United States Mechanical Construction And Facilities Services
Restructuring Cost and Reserve [Line Items]
Restructuring reserve	0	188	0
Restructuring expenses	180	519
Payments for restructuring	(368)	(331)
UNITED STATES | Operating Segments [Member] | United States Building Services
Restructuring Cost and Reserve [Line Items]
Restructuring reserve	40	13	$ 81
Restructuring expenses	945	919
Payments for restructuring	(918)	$ (987)
UNITED STATES | Operating Segments [Member] | Employee Severance [Member] | United States Electrical Construction And Facilities Services
Restructuring Cost and Reserve [Line Items]
Restructuring expenses	391
UNITED STATES | Operating Segments [Member] | Employee Severance [Member] | United States Mechanical Construction And Facilities Services
Restructuring Cost and Reserve [Line Items]
Restructuring expenses	180
UNITED STATES | Operating Segments [Member] | Employee Severance [Member] | United States Building Services
Restructuring Cost and Reserve [Line Items]
Restructuring expenses	945
UNITED STATES | Operating Segments [Member] | Facility Closing [Member] | United States Electrical Construction And Facilities Services
Restructuring Cost and Reserve [Line Items]
Restructuring expenses	61
UNITED STATES | Operating Segments [Member] | Facility Closing [Member] | United States Mechanical Construction And Facilities Services
Restructuring Cost and Reserve [Line Items]
Restructuring expenses	0
UNITED STATES | Operating Segments [Member] | Facility Closing [Member] | United States Building Services
Restructuring Cost and Reserve [Line Items]
Restructuring expenses	$ 0