Debt Obligations - Other (Details) $ in Thousands				12 Months Ended
	Dec. 31, 2019 USD ($)	Dec. 20, 2019 USD ($)	Dec. 19, 2019	Dec. 31, 2021 USD ($) item	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Debt Obligations
Payments on term loan				$ 15,000	$ 15,000
Financing and professional cost						$ 1,405
Outstanding balance				388,220	236,000
Reconciliation of Credit Facility Adjusted EBITDA to net income
Net income				143,348	150,139	114,324
Provision for income taxes				46,926	41,401	37,418
Depreciation and amortization expense				(28,400)	(27,900)	(24,500)
Stock-based compensation				$ 10,593	6,934	$ 5,878
Principal financial covenants
Fixed charge coverage ratio				2.00
Other disclosures
2023				$ 27,981
2024				35,352
Revolving credit facility
Debt Obligations
Borrowing capacity		$ 450,000
Outstanding borrowings				220,000
Outstanding balance				220,000	70,000
Letters of credit amount outstanding				53,500
Credit available				176,500
Book value of assets pledged as collateral				$ 98,900
Number of quarters of net earnings used for the calculation of the credit facility adjusted EBITDA | item				4
Principal financial covenants
Number of interest rate options | item				2
Leverage ratio				1.2
Fixed charge coverage ratio				42.0
Number of quarters of capital expenditures, tax provision, dividends and stock repurchase payments used for calculation of fixed charge coverage ratio | item				4
Period of Credit Facility Adjusted EBITDA for determining additional margins				12 months
Debt Instrument, Interest Rate, Effective Percentage [Abstract]
Weighted average interest rate (as a percent)				1.40%
Revolving credit facility | Through maturity
Principal financial covenants
Leverage ratio				3.00
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: 1.00 to 1.75
Debt Instrument, Interest Rate, Effective Percentage [Abstract]
Letter of credit fees (as a percent)				1.50%
Commitment fees payable on unused portion of the facility (as a percent)				0.25%
Revolving credit facility | Consolidated Total Indebtedness to Credit Facility Adjusted EBITDA: 1.75 to 2.50
Debt Instrument, Interest Rate, Effective Percentage [Abstract]
Letter of credit fees (as a percent)				1.75%
Commitment fees payable on unused portion of the facility (as a percent)				0.30%
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 | Minimum
Principal financial covenants
Fixed charge coverage ratio				1.50
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
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		600,000		$ 570,000
Line of credit borrowing capacity accordion option		150,000
Payments on term loan				30,000
Financing and professional cost	$ 1,400
Term loan
Debt Obligations
Borrowing capacity		$ 150,000
Repayment in terms of percentage of original aggregate principal amount		3.75%	1.25%
Payments of financing costs term loan	$ 400
Outstanding balance				$ 120,000	135,000
Debt Instrument, Interest Rate, Effective Percentage [Abstract]
Weighted average interest rate (as a percent)				1.40%
Notes to former owners
Other disclosures
Cumulative number of companies acquired | item				10
Outstanding balance				$ 47,954	$ 31,000
Promissory note | MEP Holdings
Other disclosures
Outstanding balance				$ 7,600
Stated interest rate (as a percent)				2.50%
Promissory note | Ivey
Other disclosures
Outstanding balance				$ 8,000
Stated interest rate (as a percent)				2.50%
Promissory note | Walker
Other disclosures
Outstanding balance				$ 2,000
Stated interest rate (as a percent)				4.00%
Promissory note | Amteck
Other disclosures
Outstanding balance				$ 10,000
Stated interest rate (as a percent)				2.50%
Promissory note | Immaterial acquisitions
Other disclosures
Outstanding balance				$ 20,400
Promissory note | Minimum | Immaterial acquisitions
Other disclosures
Stated interest rate (as a percent)				2.30%
Promissory note | Maximum | Immaterial acquisitions
Other disclosures
Stated interest rate (as a percent)				3.50%
Letter of Credit
Debt Obligations
Borrowing capacity		$ 160,000
Outstanding notes | Immaterial acquisitions
Other disclosures
Number of acquisitions | item				6
Federal Funds Rate | Revolving credit facility | Base rate
Market rates relating to interest options
Market rate (as a percent)				0.57%
Debt Instrument, Interest Rate, Effective Percentage [Abstract]
Additional per annum interest margin (as a percent)				0.50%
Wells Fargo Bank, N.A. Prime Rate | Revolving credit facility | Base rate
Market rates relating to interest options
Market rate (as a percent)				3.25%
One-month LIBOR | Revolving credit facility | Base rate
Market rates relating to interest options
Market rate (as a percent)				1.10%
Debt Instrument, Interest Rate, Effective Percentage [Abstract]
Additional per annum interest margin (as a percent)				1.00%
One-month LIBOR | Revolving credit facility | Eurodollar rate
Market rates relating to interest options
Variable rate basis				One-month LIBOR
Market rate (as a percent)				0.10%
Six-month LIBOR | Revolving credit facility | Eurodollar rate
Market rates relating to interest options
Variable rate basis				Six-month LIBOR
Market rate (as a percent)				0.34%