Debt Obligations - Other (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Apr. 30, 2018 USD ($)	Apr. 18, 2018 USD ($)	Apr. 17, 2018 USD ($)	Mar. 31, 2018 USD ($)
Reconciliation of Earnings before Interest Tax Depreciation and Amortization to Net Income
Net income	$ 112,903	$ 55,272	$ 64,896
Provision for income taxes	35,773	45,666	36,165
Stock-based compensation	7,161	6,377	$ 5,041
Other disclosures
Outstanding balance	73,639	59,926
Shoffner
Other disclosures
Outstanding balance	100
Acquired debt	$ 400
London Interbank Offered Rate (LIBOR) | Shoffner
Additional per annum interest margin added under:
Additional per annum interest margin (as a percent)	2.25%
Revolving credit facility
Debt Obligations
Outstanding borrowings	$ 50,000
Letters of credit amount outstanding	33,600
Credit available	316,400
Book value of assets pledged as collateral	$ 48,900
Number of quarters of net earnings used for the calculation of the credit facility adjusted EBITDA | item	4
Reconciliation of Earnings before Interest Tax Depreciation and Amortization to Net Income
Net income	$ 112,903
Provision for income taxes	35,773
Interest expense, net	(3,637)
Depreciation and amortization expense	42,689
Stock-based compensation	7,161
Pre-acquisition results of acquired companies, as defined under the Facility	(3,771)
Credit Facility Adjusted EBITDA	$ 205,934
Principal financial covenants
Number of interest rate options | item	2
Leverage ratio	0.40
Fixed charge coverage ratio	21.9
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
Additional per annum interest margin added under:
Weighted average interest rate (as a percent)	3.70%
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
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	2.00
Revolving credit facility | Minimum | Covenant Requirement
Principal financial covenants
Net leverage ratio used as basis for other restrictions	2.00
Revolving credit facility | Maximum
Principal financial covenants
Net leverage ratio after giving effect to stock repurchases for calculation of the fixed charge coverage ratio	1.75
Permitted amount of acquisitions per transaction	$ 40,000
Aggregate purchase price of current acquisition and acquisitions in the preceding 12 month period for determining permitted amount of acquisition per transaction	$ 80,000
Revolving credit facility | Maximum | Covenant Requirement
Principal financial covenants
Net leverage ratio used to determine exclusion of stock repurchases and the payment of dividends for calculation of the fixed charge coverage ratio	1.75
Revolving credit facility | Maximum | Stock Repurchases Through September 30, 2015
Principal financial covenants
Amount of stock repurchases to maintain maximum net leverage ratio	$ 30,000
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					$ 400,000	$ 325,000
Line of credit borrowing capacity accordion option	$ 100,000
Financing and professional cost				$ 800
Unamortized costs							$ 1,100
Notes to former owners
Other disclosures
Cumulative number of companies acquired | item	6
Outstanding balance	$ 26,813	$ 15,325
Subordinate note | Shoffner
Other disclosures
Outstanding balance	$ 500
Weighted average interest rate (as a percent)	3.00%
Subordinate note | Acquisition First Quarter 2018 [Member]
Other disclosures
Outstanding balance	$ 1,000
Weighted average interest rate (as a percent)	2.50%
Subordinate note | Acquisition Second Quarter 2018 [Member]
Other disclosures
Outstanding balance	$ 1,000
Weighted average interest rate (as a percent)	3.50%
Promissory note | BCH
Other disclosures
Outstanding balance	$ 14,300
Weighted average interest rate (as a percent)	3.00%
Promissory note | Acquisition Third Quarter 2018 [Member]
Other disclosures
Outstanding balance	$ 6,500
Weighted average interest rate (as a percent)	3.50%
Promissory note | Acquisition Two Third Quarter 2018 [Member]
Other disclosures
Outstanding balance	$ 3,500
Weighted average interest rate (as a percent)	2.90%
Federal Funds Rate | Revolving credit facility | Base Rate
Market rates relating to interest options
Market rate (as a percent)	2.95%
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)	5.50%
One-month LIBOR | Revolving credit facility | Base Rate
Market rates relating to interest options
Market rate (as a percent)	3.52%
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)	2.52%
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)	2.87%