Debt Obligations - Other (Details) $ in Thousands				12 Months Ended
	Dec. 31, 2019 USD ($) item	Dec. 20, 2019 USD ($)	Dec. 19, 2019 USD ($)	Dec. 31, 2019 USD ($) item	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Debt Obligations
Financing and professional cost				$ 1,405	$ 844
Outstanding balance	$ 226,483			226,483	76,918
Reconciliation of Credit Facility Adjusted EBITDA to net income
Net income				114,324	112,903	$ 55,272
Provision for income taxes				37,418	35,773	45,666
Stock-based compensation				$ 5,878	7,161	$ 6,377
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	28,000			$ 28,000
Outstanding balance	28,000			28,000	50,000
Letters of credit amount outstanding	40,900			40,900
Credit available	381,100			381,100
Book value of assets pledged as collateral	$ 58,200			$ 58,200
Number of quarters of net earnings used for the calculation of the credit facility adjusted EBITDA | item				4
Reconciliation of Credit Facility Adjusted EBITDA to net income
Net income				$ 114,324
Provision for income taxes				37,418
Interest expense, net				9,093
Depreciation and amortization expense				51,572
Stock-based compensation				5,878
Pre-acquisition results of acquired companies, as defined under the Facility				(1,404)
Credit Facility Adjusted EBITDA				$ 219,689
Principal financial covenants
Number of interest rate options | item				2
Leverage ratio	1.0			1.0
Fixed charge coverage ratio	11.3			11.3
Period of Credit Facility Adjusted EBITDA for determining additional margins				12 months
Additional per annum interest margin added under:
Weighted average interest rate (as a percent)	3.20%			3.20%
Revolving credit facility | Through maturity
Principal financial covenants
Leverage ratio	3.00			3.00
Revolving credit facility | Consolidated Total Indebtedness to Credit Facility Adjusted EBITDA: Less than 1.00
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 | Consolidated Total Indebtedness to Credit Facility Adjusted EBITDA: 1.00 to 1.75
Additional per annum interest margin added under:
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
Additional per annum interest margin added under:
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
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 | Minimum
Principal financial covenants
Fixed charge coverage ratio	1.50			1.50
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
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
Line of credit borrowing capacity accordion option		150,000
Unamortized costs			$ 1,300
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	$ 150,000			$ 150,000
Additional per annum interest margin added under:
Weighted average interest rate (as a percent)	3.30%			3.30%
Notes to former owners
Other disclosures
Cumulative number of companies acquired | item	6			6
Outstanding balance	$ 48,483			$ 48,483	26,813
Promissory note | Walker
Other disclosures
Outstanding balance	$ 25,000			$ 25,000
Stated interest rate (as a percent)	4.00%			4.00%
Promissory note | BCH
Other disclosures
Outstanding balance	$ 14,300			$ 14,300
Weighted average interest rate (as a percent)	3.00%			3.00%
Promissory note | Four Immaterial Acquisition
Other disclosures
Outstanding balance	$ 9,200			$ 9,200
Promissory note | Minimum | Five Immaterial Acquisition
Other disclosures
Weighted average interest rate (as a percent)	2.90%			2.90%
Promissory note | Maximum | Five Immaterial Acquisition
Other disclosures
Weighted average interest rate (as a percent)	3.50%			3.50%
Letter of Credit
Debt Obligations
Borrowing capacity		$ 160,000
Other debt
Debt Obligations
Outstanding balance					$ 105
Federal Funds Rate | Revolving credit facility | Base Rate
Market rates relating to interest options
Market rate (as a percent)	2.05%			2.05%
Additional per annum interest margin added under:
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)	4.75%			4.75%
One-month LIBOR | Revolving credit facility | Base Rate
Market rates relating to interest options
Market rate (as a percent)	2.76%			2.76%
Additional per annum interest margin added under:
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)	1.76%			1.76%
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)	1.91%			1.91%