Goodwill and Identifiable Intangible Assets, Net - Changes in Carrying Amount of Goodwill (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2019	Dec. 31, 2018
Changes in the carrying amount of goodwill
Balance at beginning of year	$ 235,182	$ 200,584
Additions (See Note 4)	97,018	34,598
Balance at end of period	332,200	235,182
Mechanical Services
Changes in the carrying amount of goodwill
Balance at beginning of year	235,182	200,584
Additions (See Note 4)	579	34,598
Impact of segment reorganization	(1,101)
Balance at end of period	234,660	$ 235,182
Electrical Services
Changes in the carrying amount of goodwill
Additions (See Note 4)	96,439
Impact of segment reorganization	1,101
Balance at end of period	$ 97,540

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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