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Goodwill and Identifiable Intangible Assets, Net - Changes in Carrying Amount of Goodwill (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Changes in the carrying amount of goodwill
Balance at beginning of year		$ 235,182	$ 200,584
Acquisitions and purchase price adjustments (See Note 4)		97,265	34,598
Impairment adjustment	$ (1,100)			$ (1,105)
Balance at end of period		332,447	235,182	200,584
Mechanical Services Segment
Changes in the carrying amount of goodwill
Accumulated impairment charges		116,600	116,600
Mechanical Services
Changes in the carrying amount of goodwill
Balance at beginning of year		235,182	200,584
Acquisitions and purchase price adjustments (See Note 4)		579	34,598
Impact of segment reorganization		(1,101)
Balance at end of period		234,660	$ 235,182	$ 200,584
Electrical Services
Changes in the carrying amount of goodwill
Acquisitions and purchase price adjustments (See Note 4)		96,686
Impact of segment reorganization		1,101
Balance at end of period		$ 97,787