Long Term Debt Obligations-Other (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) item	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Long-Term Debt Obligations
Line of credit borrowing capacity accordion option	$ 100,000								$ 100,000
Reconciliation of Earnings before Interest Tax Depreciation and Amortization to Net Income
Net income including noncontrolling interests	7,539	$ 22,284	$ 17,972	$ 7,477	$ 16,867	$ 20,471	$ 17,717	$ 9,841	55,272	$ 64,896	$ 57,440
Provision for income taxes									(45,666)	(36,165)	(31,224)
Stock-based compensation									6,377	5,041	$ 5,609
Goodwill impairment				$ (1,100)					(1,105)
Shoffner
Other disclosures
Outstanding balance	200								200
Debt acquired related to coil line duct processing system	400								400
Current debt	100								$ 100
London Interbank Offered Rate (LIBOR) | Shoffner
Market rates relating to interest options
Interest rate margin (as a percent)									2.25%
Additional per annum interest margin added under:
Additional per annum interest margin (as a percent)									2.25%
Revolving credit facility
Long-Term Debt Obligations
Borrowing capacity	325,000								$ 325,000
Outstanding borrowings	45,000								45,000
Letters of credit amount outstanding	39,600								39,600
Credit available	240,400								240,400
Book value of assets pledged as collateral	$ 40,900								$ 40,900
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 including noncontrolling interests									$ 55,272
Provision for income taxes									(45,666)
Interest expense, net									(3,086)
Depreciation and amortization expense									37,456
Stock-based compensation									6,377
Goodwill impairment									(1,105)
Pre-acquisition results of acquired companies, as defined under the Facility									(4,597)
Credit Facility Adjusted EBITDA									$ 153,559
Principal financial covenants
Leverage ratio	0.40								0.40
Fixed charge coverage ratio	21.8								21.8
Number of quarters of capital expenditures, tax provision, dividends and stock repurchase payments used for calculation of fixed charge coverage ratio | item									4
Number of interest rate options | item									2
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)	2.80%								2.80%
Other disclosures
Outstanding balance	$ 45,000								$ 45,000
Revolving credit facility | Through maturity
Principal financial covenants
Leverage ratio	2.75								2.75
Revolving credit facility | Consolidated Total Indebtedness to Credit Facility Adjusted EBITDA: Less than 0.75
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: 0.75 to 1.50
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: 1.50 to 2.25
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: 2.25 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								2.00
Revolving credit facility | Minimum | Covenant Requirement
Principal financial covenants
Net leverage ratio used as basis for other restrictions	2.00								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	1.50								1.50
Permitted amount of acquisitions per transaction	$ 30,000								$ 30,000
Aggregate purchase price of current acquisition and acquisitions in the preceding 12 month period for determining permitted amount of acquisition per transaction	$ 65,000								$ 65,000
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	1.50								1.50
Revolving credit facility | Maximum | Stock Repurchases Through September 30, 2015
Principal financial covenants
Amount of stock repurchases to maintain maximum net leverage ratio									$ 25,000
Revolving credit facility | Maximum | Stock Repurchases After February 22, 2016 And On Or Before December 31, 2017
Principal financial covenants
Amount of stock repurchases to maintain maximum net leverage ratio									$ 25,000
Revolving credit facility | Base rate | Consolidated Total Indebtedness to Credit Facility Adjusted EBITDA: Less than 0.75
Market rates relating to interest options
Interest rate margin (as a percent)									0.25%
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: 0.75 to 1.50
Market rates relating to interest options
Interest rate margin (as a percent)									0.50%
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: 1.50 to 2.25
Market rates relating to interest options
Interest rate margin (as a percent)									0.75%
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: 2.25 or greater
Market rates relating to interest options
Interest rate margin (as a percent)									1.00%
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 0.75
Market rates relating to interest options
Interest rate margin (as a percent)									1.25%
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: 0.75 to 1.50
Market rates relating to interest options
Interest rate margin (as a percent)									1.50%
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: 1.50 to 2.25
Market rates relating to interest options
Interest rate margin (as a percent)									1.75%
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: 2.25 or greater
Market rates relating to interest options
Interest rate margin (as a percent)									2.00%
Additional per annum interest margin added under:
Additional per annum interest margin (as a percent)									2.00%
Notes to former owners
Other disclosures
Outstanding balance	$ 15,325				2,250				$ 15,325	2,250
Subordinate note | Shoffner
Other disclosures
Outstanding balance	$ 1,000								$ 1,000
Weighted average interest rate (as a percent)	3.00%								3.00%
Promissory note | BCH
Other disclosures
Outstanding balance	$ 14,300								$ 14,300
Weighted average interest rate (as a percent)	3.00%								3.00%
Other debt
Other disclosures
Outstanding balance	$ 214				$ 305				$ 214	$ 305
Federal Funds Rate | Revolving credit facility | Base rate
Market rates relating to interest options
Interest rate margin (as a percent)									0.50%
Market rate (as a percent)	1.87%								1.87%
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)	4.50%								4.50%
One-month LIBOR | Revolving credit facility | Base rate
Market rates relating to interest options
Interest rate margin (as a percent)									1.00%
Market rate (as a percent)	2.56%								2.56%
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)	1.56%								1.56%
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)	1.84%								1.84%