Debt Obligations - Other (Details) $ in Thousands				3 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 20, 2019	Dec. 19, 2019 USD ($)	Mar. 31, 2020 USD ($) item	Mar. 31, 2019 USD ($)
Debt Obligations
Outstanding balance	$ 226,483			$ 334,292
Reconciliation of Credit Facility Adjusted EBITDA to net income
Net income				17,716	$ 19,866
Provision for income taxes				6,751	6,933
Stock-based compensation				$ 3,631	$ 3,176
Principal financial covenants
Fixed charge coverage ratio				2.00
Revolving credit facility
Debt Obligations
Borrowing capacity	450,000
Payments of financing costs line of credit arrangements	1,000
Outstanding borrowings				$ 150,000
Outstanding balance	28,000			150,000
Letters of credit amount outstanding				55,600
Credit available				$ 244,400
Principal financial covenants
Number of interest rate options | item				2
Leverage ratio				1.5
Fixed charge coverage ratio				10.7
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)				2.50%
Revolving credit facility | Through maturity
Principal financial covenants
Leverage ratio				3.00
Revolving credit facility | Minimum
Principal financial covenants
Fixed charge coverage ratio				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
Revolving credit facility | Maximum
Principal financial covenants
Permitted amount of acquisitions per transaction				$ 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
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			$ 140,625
Other disclosures
Weighted average interest rate (as a percent)				2.50%
Notes to former owners
Other disclosures
Cumulative number of companies acquired | item				6
Outstanding balance	48,483			$ 43,667
Promissory note | Walker
Other disclosures
Outstanding balance				$ 25,000
Weighted average interest rate (as a percent)				4.00%
Promissory note | BCH
Other disclosures
Outstanding balance				$ 7,200
Weighted average interest rate (as a percent)				3.00%
Promissory note | Five Immaterial Acquisition
Other disclosures
Outstanding balance				$ 3,500
Promissory note | Electrical Contractor North Carolina
Other disclosures
Outstanding balance				$ 8,000
Weighted average interest rate (as a percent)				3.00%
Promissory note | Minimum | Five Immaterial Acquisition
Other disclosures
Weighted average interest rate (as a percent)				3.00%
Promissory note | Maximum | Five Immaterial Acquisition
Other disclosures
Weighted average interest rate (as a percent)				3.50%
Letter of Credit
Debt Obligations
Borrowing capacity	$ 160,000