Goodwill and Identifiable Intangible Assets - Narrative (Details) - USD ($)		12 Months Ended
	Oct. 01, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Goodwill and Intangible Assets[Line Items]
Goodwill		$ 990,887,000	$ 964,893,000	$ 979,628,000
Goodwill acquired		25,900,000	40,700,000
Non-cash goodwill impairment charge		0	57,451,000	0
Non-cash expense for impairment of identifiable intangible assets		907,000	57,819,000	2,428,000
Non-cash impairment charge of finite-lived intangible assets		900,000.0	0	0
Accumulated impairment charge				210,600,000
Indefinite-lived intangible asset acquired		$ 36,600,000	56,600,000
Amortization periods		8 years 9 months
Amortization of identifiable intangible assets		$ 42,443,000	48,594,000	40,908,000
Domestic Segments
Goodwill and Intangible Assets[Line Items]
Perpetual growth rate used for annual testing	2.70%
Domestic Construction
Goodwill and Intangible Assets[Line Items]
Weighted average cost of capital used in annual testing for impairment	9.80%
United States Electrical Construction And Facilities Services
Goodwill and Intangible Assets[Line Items]
Goodwill		$ 133,207,000	125,707,000	123,600,000
Goodwill allocated		13.40%
Effect of 50 Basis Point Increase in the Weighted Average Costs of Capital		$ 91,700,000
Effect of 50 Basis Point Decrease in the Perpetual Growth Rate		52,500,000
Non-cash goodwill impairment charge			0
United States Mechanical Construction And Facilities Services
Goodwill and Intangible Assets[Line Items]
Goodwill		$ 256,321,000	256,265,000	226,596,000
Goodwill allocated		25.90%
Effect of 50 Basis Point Increase in the Weighted Average Costs of Capital		$ 132,400,000
Effect of 50 Basis Point Decrease in the Perpetual Growth Rate		74,100,000
Non-cash goodwill impairment charge			0
United States Building Services
Goodwill and Intangible Assets[Line Items]
Goodwill		$ 274,171,000	255,733,000	244,793,000
Goodwill allocated		27.70%
Weighted average cost of capital used in annual testing for impairment	9.70%
Effect of 50 Basis Point Increase in the Weighted Average Costs of Capital		$ 71,200,000
Effect of 50 Basis Point Decrease in the Perpetual Growth Rate		38,300,000
Non-cash goodwill impairment charge			0
Accumulated impairment charge				139,500,000
United States Industrial Services
Goodwill and Intangible Assets[Line Items]
Goodwill		$ 327,188,000	327,188,000	384,639,000
Goodwill allocated		33.00%
Weighted average cost of capital used in annual testing for impairment	10.50%
Effect of 50 Basis Point Increase in the Weighted Average Costs of Capital		$ 46,400,000
Effect of 50 Basis Point Decrease in the Perpetual Growth Rate		$ 23,300,000
Non-cash goodwill impairment charge			57,451,000
Accumulated impairment charge				71,100,000
Contract backlog
Goodwill and Intangible Assets[Line Items]
Amortization periods		6 months
Developed technology/Vendor network
Goodwill and Intangible Assets[Line Items]
Amortization periods		8 years 6 months
Customer relationships
Goodwill and Intangible Assets[Line Items]
Amortization periods		8 years 9 months
Trade names (amortized)
Goodwill and Intangible Assets[Line Items]
Amortization periods		11 years 9 months
Trade names (unamortized)
Goodwill and Intangible Assets[Line Items]
Non-cash expense for impairment of identifiable intangible assets		$ 0	$ 300,000	$ 2,400,000