Debt Obligations - Other (Details) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021 USD ($) item	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2021 USD ($) item	Sep. 30, 2020 USD ($)	Aug. 01, 2021 USD ($)	Dec. 31, 2020 USD ($)
Debt Obligations
Payments on term loan							$ 15,000	$ 15,000
Outstanding balance	$ 272,938						272,938			$ 236,000
Reconciliation of Credit Facility Adjusted EBITDA to net income
Net income	46,297	$ 32,965	$ 26,491	$ 50,088	$ 39,495	$ 17,716	105,753	107,299
Provision for income taxes	14,999			$ 8,279			33,553	30,100
Stock-based compensation							7,378	$ 5,949
Amteck
Other disclosures
Acquired debt									$ 785
Revolving credit facility
Debt Obligations
Borrowing capacity	450,000						450,000
Outstanding borrowings	115,000						115,000
Outstanding balance	115,000						115,000			70,000
Letters of credit amount outstanding	49,500						49,500
Credit available	$ 285,500						$ 285,500
Principal financial covenants
Number of interest rate options | item							2
Other disclosures
Weighted average interest rate (as a percent)	1.30%						1.30%
Revolving credit facility | Consolidated Total Indebtedness to Credit Facility Adjusted EBITDA: Less than 0.75
Debt Instrument, Interest Rate, Effective Percentage [Abstract]
Letter of credit fees (as a percent)							1.25%
Commitment fees payable on unused portion of the facility (as a percent)							0.20%
Revolving credit facility | Consolidated Total Indebtedness to Credit Facility Adjusted EBITDA: 2.25 or greater
Debt Instrument, Interest Rate, Effective Percentage [Abstract]
Letter of credit fees (as a percent)							2.00%
Commitment fees payable on unused portion of the facility (as a percent)							0.35%
Revolving credit facility | Base Rate | Consolidated Total Indebtedness to Credit Facility Adjusted EBITDA: Less than 0.75
Debt Instrument, Interest Rate, Effective Percentage [Abstract]
Additional per annum interest margin (as a percent)							0.25%
Revolving credit facility | Base Rate | Consolidated Total Indebtedness to Credit Facility Adjusted EBITDA: 0.75 to 1.50
Debt Instrument, Interest Rate, Effective Percentage [Abstract]
Additional per annum interest margin (as a percent)							0.50%
Revolving credit facility | Base Rate | Consolidated Total Indebtedness to Credit Facility Adjusted EBITDA: 1.50 to 2.25
Debt Instrument, Interest Rate, Effective Percentage [Abstract]
Additional per annum interest margin (as a percent)							0.75%
Revolving credit facility | Base Rate | Consolidated Total Indebtedness to Credit Facility Adjusted EBITDA: 2.25 or greater
Debt Instrument, Interest Rate, Effective Percentage [Abstract]
Additional per annum interest margin (as a percent)							1.00%
Revolving credit facility | Eurodollar rate | Consolidated Total Indebtedness to Credit Facility Adjusted EBITDA: Less than 0.75
Debt Instrument, Interest Rate, Effective Percentage [Abstract]
Additional per annum interest margin (as a percent)							1.25%
Revolving credit facility | Eurodollar rate | Consolidated Total Indebtedness to Credit Facility Adjusted EBITDA: 0.75 to 1.50
Debt Instrument, Interest Rate, Effective Percentage [Abstract]
Additional per annum interest margin (as a percent)							1.50%
Revolving credit facility | Eurodollar rate | Consolidated Total Indebtedness to Credit Facility Adjusted EBITDA: 1.50 to 2.25
Debt Instrument, Interest Rate, Effective Percentage [Abstract]
Additional per annum interest margin (as a percent)							1.75%
Revolving credit facility | Eurodollar rate | Consolidated Total Indebtedness to Credit Facility Adjusted EBITDA: 2.25 or greater
Debt Instrument, Interest Rate, Effective Percentage [Abstract]
Additional per annum interest margin (as a percent)							2.00%
Amended senior revolving credit facility
Debt Obligations
Borrowing capacity	$ 570,000						$ 570,000			600,000
Line of credit borrowing capacity accordion option	150,000						150,000
Payments on term loan							30,000
Term loan
Debt Obligations
Borrowing capacity	150,000						150,000
Outstanding balance	$ 120,000						$ 120,000			135,000
Other disclosures
Weighted average interest rate (as a percent)	1.30%						1.30%
Notes to former owners
Other disclosures
Cumulative number of companies acquired | item	6						6
Outstanding balance	$ 35,500						$ 35,500			$ 31,000
Promissory note | Walker
Other disclosures
Outstanding balance	$ 6,000						$ 6,000
Weighted average interest rate (as a percent)	4.00%						4.00%
Promissory note | TAS Energy
Other disclosures
Outstanding balance	$ 4,000						$ 4,000
Weighted average interest rate (as a percent)	3.50%						3.50%
Promissory note | T E C Industrial Construction and Maintenance
Other disclosures
Outstanding balance	$ 7,000						$ 7,000
Weighted average interest rate (as a percent)	2.50%						2.50%
Promissory note | Electrical Contractor North Carolina
Other disclosures
Outstanding balance	$ 5,000						$ 5,000
Weighted average interest rate (as a percent)	3.00%						3.00%
Promissory note | Amteck
Other disclosures
Outstanding balance	$ 10,000						$ 10,000
Weighted average interest rate (as a percent)	2.50%						2.50%
Promissory note | Mechanical Contractor Utah
Other disclosures
Outstanding balance	$ 3,500						$ 3,500
Weighted average interest rate (as a percent)	2.50%						2.50%
Letter of Credit
Debt Obligations
Borrowing capacity	$ 160,000						$ 160,000