Debt Obligations - Other (Details) $ in Thousands	3 Months Ended			9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) item	Jun. 30, 2018 USD ($) item	Mar. 31, 2018 item	Sep. 30, 2018 USD ($) item	Dec. 31, 2017 USD ($) item	Apr. 18, 2018 USD ($)	Apr. 17, 2018 USD ($)
Other disclosures
Number of Businesses Acquired | item	2	3	2		4
Outstanding balance	$ 96,951			$ 96,951	$ 60,539
Outstanding balance	93,672			93,672	59,926
Shoffner
Other disclosures
Outstanding balance	100			100
Debt acquired related to coil line duct processing system	400			400
Current debt	100			$ 100
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
Long-Term Debt Obligations
Outstanding borrowings	70,000			$ 70,000
Letters of credit amount outstanding	33,600			33,600
Credit available	$ 296,400			$ 296,400
Principal financial covenants
Number of interest rate options | item				2
Leverage ratio	0.50			0.50
Fixed charge coverage ratio	18.6			18.6
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
Outstanding balance	$ 70,000			$ 70,000	45,000
Weighted average interest rate (as a percent)	3.40%			3.40%
Revolving credit facility | Through maturity
Principal financial covenants
Leverage ratio	3.00			3.00
Revolving credit facility | Minimum
Principal financial covenants
Fixed charge coverage ratio	2.00			2.00
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 | 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	1.75			1.75
Permitted amount of acquisitions per transaction	$ 40,000			$ 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			$ 80,000
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 | 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			1.75
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				$ 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
Long-Term Debt Obligations
Borrowing capacity						$ 400,000	$ 325,000
Line of credit borrowing capacity accordion option	$ 100,000			$ 100,000
Financing and professional cost	800			800
Unamortized costs	1,100			$ 1,100
Notes to former owners
Other disclosures
Number of Businesses Acquired | item				6
Outstanding balance	26,813			$ 26,813	15,325
Subordinate note | Shoffner
Other disclosures
Outstanding balance	$ 500			$ 500
Weighted average interest rate (as a percent)	3.00%			3.00%
Subordinate note | Acquisition First Quarter 2018 [Member]
Other disclosures
Outstanding balance	$ 1,000			$ 1,000
Weighted average interest rate (as a percent)	2.50%			2.50%
Subordinate note | Acquisition Second Quarter 2018 [Member]
Other disclosures
Outstanding balance		$ 1,000
Weighted average interest rate (as a percent)	3.50%			3.50%
Promissory note | BCH
Other disclosures
Outstanding balance	$ 14,300			$ 14,300
Weighted average interest rate (as a percent)	3.00%			3.00%
Promissory note | Acquisition Third Quarter 2018 [Member]
Other disclosures
Outstanding balance	$ 6,500			$ 6,500
Weighted average interest rate (as a percent)	3.50%			3.50%
Promissory note | Acquisition Two Third Quarter 2018 [Member]
Other disclosures
Outstanding balance	$ 3,500			$ 3,500
Weighted average interest rate (as a percent)	2.90%			2.90%
Other debt
Other disclosures
Outstanding balance	$ 138			$ 138	$ 214