Goodwill and Identifiable Intangible Assets, Net - Changes in Carrying Amount of Goodwill (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2021	Dec. 31, 2020
Changes in the carrying amount of goodwill
Balance at beginning of year	$ 464,392	$ 332,447
Acquisitions and purchase price adjustments (See Note 5)	62,572	131,945
Balance at end of period	526,964	464,392
Mechanical Services Segment
Changes in the carrying amount of goodwill
Balance at beginning of year	307,448	234,660
Acquisitions and purchase price adjustments (See Note 5)	8,089	72,788
Impact of segment reorganization	1,101
Balance at end of period	316,638	307,448
Electrical Services Industry
Changes in the carrying amount of goodwill
Balance at beginning of year	156,944	97,787
Acquisitions and purchase price adjustments (See Note 5)	54,483	59,157
Impact of segment reorganization	(1,101)
Balance at end of period	$ 210,326	$ 156,944

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

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Amount of increase (decrease), classified as other, of an asset representing the future economic benefits from other assets acquired in a business combination that are not individually identified and separately recognized.

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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