Debt Obligations - Other (Details) $ in Thousands				3 Months Ended				6 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 20, 2019	Dec. 19, 2019 USD ($)	Jun. 30, 2020 USD ($) item	Mar. 31, 2020 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2020 USD ($) item	Jun. 30, 2019 USD ($)
Debt Obligations
Outstanding balance	$ 226,483			$ 231,835				$ 231,835
Reconciliation of Credit Facility Adjusted EBITDA to net income
Net income				39,495	$ 17,716	$ 24,173	$ 19,866	57,211	$ 44,039
Provision for income taxes				$ 15,070		$ 6,933		21,821	13,866
Stock-based compensation								$ 5,188	$ 4,679
Principal financial covenants
Fixed charge coverage ratio				2.00				2.00
Revolving credit facility
Debt Obligations
Borrowing capacity	450,000
Payments of financing costs line of credit arrangements	1,000
Outstanding borrowings				$ 55,000				$ 55,000
Outstanding balance	28,000			55,000				55,000
Letters of credit amount outstanding				55,600				55,600
Credit available				$ 339,400				$ 339,400
Principal financial covenants
Number of interest rate options | item								2
Leverage ratio				0.9				0.9
Fixed charge coverage ratio				7.3				7.3
Number of quarters of capital expenditures, tax provision, dividends and stock repurchase payments used for calculation of fixed charge coverage ratio | item								4
Other disclosures
Weighted average interest rate (as a percent)				2.20%				2.20%
Revolving credit facility | Through maturity
Principal financial covenants
Leverage ratio				3.00				3.00
Revolving credit facility | Minimum
Principal financial covenants
Fixed charge coverage ratio				1.50				1.50
Additional per annum interest margin added under:
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 | Minimum | Covenant Requirement
Principal financial covenants
Net leverage ratio used as basis for other restrictions				2.50				2.50
Revolving credit facility | Maximum
Principal financial covenants
Permitted amount of acquisitions per transaction				$ 5,000				$ 5,000
Aggregate purchase price of current acquisition and acquisitions in the preceding 12 month period for determining permitted amount of acquisition per transaction				10,000				$ 10,000
Additional per annum interest margin added under:
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
Additional per annum interest margin added under:
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
Additional per annum interest margin added under:
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
Additional per annum interest margin added under:
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
Additional per annum interest margin added under:
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
Additional per annum interest margin added under:
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
Additional per annum interest margin added under:
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
Additional per annum interest margin added under:
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
Additional per annum interest margin added under:
Additional per annum interest margin (as a percent)								2.00%
Amended senior revolving credit facility
Debt Obligations
Borrowing capacity	600,000		$ 400,000
Repayment in terms of percentage of original aggregate principal amount		3.75%	1.25%
Line of credit borrowing capacity accordion option	150,000
Financing and professional cost	1,400
Unamortized costs			$ 1,300
Term loan
Debt Obligations
Borrowing capacity	150,000
Payments of financing costs term loan	400
Outstanding balance	150,000			$ 138,750				$ 138,750
Other disclosures
Weighted average interest rate (as a percent)				1.70%				1.70%
Notes to former owners
Other disclosures
Cumulative number of companies acquired | item				5				5
Outstanding balance	48,483			$ 38,050				$ 38,050
Promissory note | Walker
Other disclosures
Outstanding balance				$ 15,000				$ 15,000
Weighted average interest rate (as a percent)				4.00%				4.00%
Amount due in 2022				$ 2,500				$ 2,500
Amount due in 2023				12,500				12,500
Promissory note | TAS Energy
Other disclosures
Outstanding balance				$ 14,000				$ 14,000
Weighted average interest rate (as a percent)				3.50%				3.50%
Promissory note | Electrical Contractor North Carolina
Other disclosures
Outstanding balance				$ 8,000				$ 8,000
Weighted average interest rate (as a percent)				3.00%				3.00%
Promissory note | Two Immaterial Acquisitions
Other disclosures
Outstanding balance				$ 1,100				$ 1,100
Promissory note | Minimum | Two Immaterial Acquisitions
Other disclosures
Weighted average interest rate (as a percent)				3.00%				3.00%
Promissory note | Maximum | Two Immaterial Acquisitions
Other disclosures
Weighted average interest rate (as a percent)				3.50%				3.50%
Letter of Credit
Debt Obligations
Borrowing capacity	$ 160,000
Other debt
Other disclosures
Acquired debt				$ 35				$ 35