Acquisitions (Details) - USD ($) $ in Thousands	Aug. 01, 2021	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Recognized amounts of identifiable assets acquired and liabilities assumed:
Goodwill		$ 526,964	$ 464,392	$ 332,447
Amteck
Consideration transferred:
Cash paid at closing	$ 107,355
Working capital adjustment	6,892
Notes issued to former owners	10,000
Estimated fair value of contingent earn-out payments	12,900
Total consideration transferred	137,147
Recognized amounts of identifiable assets acquired and liabilities assumed:
Cash and cash equivalents	13,554
Billed and unbilled accounts receivable	43,248
Other current assets	3,422
Property and equipment	3,891
Goodwill	54,554
Identifiable intangible assets	57,700
Operating lease right-of-use asset	8,997
Accounts payable	(14,489)
Billings in excess of costs and estimated earnings	(15,199)
Current maturities of long-term debt	(1,860)
Current operating lease liabilities	(1,115)
Accrued expenses and other current liabilities	(6,889)
Long-term debt	(785)
Long-term operating lease liabilities	(7,882)
Total assets acquired and liabilities assumed	$ 137,147