Debt Obligations - Other (Details) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2021 USD ($) item	Mar. 31, 2021 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Jun. 30, 2021 USD ($) item	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Debt Obligations
Payments on term loan					$ 15,000	$ 11,250
Outstanding balance	$ 160,735				160,735		$ 236,000
Reconciliation of Credit Facility Adjusted EBITDA to net income
Net income	32,965	$ 26,491	$ 39,495	$ 17,716	59,456	57,211
Provision for income taxes	9,817		$ 15,070		18,554	21,821
Stock-based compensation					6,860	$ 5,188
Revolving credit facility
Debt Obligations
Borrowing capacity	450,000				450,000
Outstanding borrowings	15,000				15,000
Outstanding balance	15,000				15,000		70,000
Letters of credit amount outstanding	49,500				49,500
Credit available	$ 385,500				$ 385,500
Principal financial covenants
Number of interest rate options | item					2
Other disclosures
Weighted average interest rate (as a percent)	1.40%				1.40%
Revolving credit facility | Consolidated Total Indebtedness to Credit Facility Adjusted EBITDA: Less than 1.00
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: 2.50 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 1.00
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: 1.00 to 1.75
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: 1.75 to 2.50
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: 2.50 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 1.00
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: 1.00 to 1.75
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: 1.75 to 2.50
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: 2.50 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)	1.40%				1.40%
Notes to former owners
Other disclosures
Cumulative number of companies acquired | item	5				5
Outstanding balance	$ 25,735				$ 25,735		$ 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	$ 4,200				$ 4,200
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 | 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