Segments Information - Summary of Adjusted EBITDA Business Segment Information (Details) - USD ($) $ in Thousands			3 Months Ended		12 Months Ended
		Jan. 01, 2024	Dec. 31, 2024	Mar. 31, 2024	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Adjusted EBITDA attributable to Parsons Corporation
Revenue			$ 1,734,317	$ 1,535,676	$ 6,750,576	$ 5,442,749	$ 4,195,272
Direct cost of contracts					(5,344,154)	(4,236,735)	(3,248,550)
Selling, general and administrative expenses	[1]				(304,186)	(259,254)	(228,227)
Equity in earnings (losses) of unconsolidated joint ventures					(23,361)	(47,751)	16,347
Other segment items	[2]				(530,636)	(481,974)	(412,453)
Adjusted EBITDA attributable to Parsons Corporation					548,239	417,035	322,389
Adjusted EBITDA attributable to noncontrolling interests					56,714	47,638	30,393
Depreciation and amortization					(99,251)	(119,973)	(120,501)
Interest expense, net					(40,154)	(29,306)	(22,219)
Income tax expense			(18,729)	(13,324)	(76,986)	(56,138)	(39,657)
Equity-based compensation expense					(61,492)	(36,151)	(24,354)
Convertible debt repurchase loss		$ (18,400)		(18,355)	(18,355)	0	0
Transaction-related costs	[3]				(17,138)	(12,013)	(16,270)
Restructuring expense	[4]					(1,244)	(213)
Other	[5]				(912)	(1,933)	(3,003)
Net income including noncontrolling interests					290,665	207,915	126,565
Net income attributable to noncontrolling interests					(55,612)	(46,766)	(29,901)
Net income attributable to Parsons Corporation			54,180	39,750	235,053	161,149	96,664
Federal Solution Segment
Adjusted EBITDA attributable to Parsons Corporation
Revenue			1,003,323	909,608	4,007,114	3,020,701	2,212,987
Direct cost of contracts					(3,187,829)	(2,375,350)	(1,703,297)
Selling, general and administrative expenses	[1]				(157,442)	(129,507)	(117,759)
Equity in earnings (losses) of unconsolidated joint ventures					3,254	4,190	3,677
Other segment items	[2]				(249,759)	(230,784)	(196,604)
Adjusted EBITDA attributable to Parsons Corporation					415,338	289,250	199,004
Critical Infrastructure
Adjusted EBITDA attributable to Parsons Corporation
Revenue			$ 730,994	$ 626,068	2,743,462	2,422,048	1,982,285
Direct cost of contracts					(2,156,325)	(1,861,385)	(1,545,253)
Selling, general and administrative expenses	[1]				(146,744)	(129,747)	(110,468)
Equity in earnings (losses) of unconsolidated joint ventures					(26,615)	(51,941)	12,670
Other segment items	[2]				(280,877)	(251,190)	(215,849)
Adjusted EBITDA attributable to Parsons Corporation					132,901	127,785	123,385
Net income attributable to Parsons Corporation					$ (47,651)	$ 26,261	$ 0

[1] The amount of selling, general and administrative expenses (“SG&A”) is total SG&A excluding allocations.

[2] The amount of other segment items is the difference between segment revenue less direct cost of contracts, segment SG&A expenses, equity in earnings (losses) of unconsolidated joint ventures, and Adjusted EBITDA attributable to Parsons Corporation. Other segment items primarily include: i. Corporate and shared segment SG&A (excluding Adjusted EBITDA items) ii. Noncontrolling interests attributable to operating income and other income/expense iii. Bad debt expense iv. Sublease income v. Foreign currency gain/loss, and vi. Certain other income/expense items

[3] Reflects costs incurred in connection with acquisitions, and other non-recurring transaction costs, primarily fees paid for professional services and employee retention.

[4] Reflects costs associated with and related to our corporate restructuring initiatives.

[5] Includes a combination of gain/loss related to sale of fixed assets, software implementation costs, and other individually insignificant items that are non-recurring in nature.