Debt Obligations-Other (Details) $ in Thousands			3 Months Ended			9 Months Ended
	Jun. 30, 2017 item	Apr. 01, 2017 USD ($)	Sep. 30, 2017 USD ($) item	Jun. 30, 2017 item	Mar. 31, 2016 item	Sep. 30, 2017 USD ($) item	Dec. 31, 2016 USD ($)
Long-Term Debt Obligations
Line of credit borrowing capacity accordion option			$ 100,000			$ 100,000
Other disclosures
Current portion of capital lease obligations			$ 90			90	$ 163
Number of acquisitions | item			1	3	2
Shoffner
Other disclosures
Outstanding balance			$ 200			200
Debt acquired related to coil line duct processing system			400			400
Current debt			100			$ 100
BCH
Other disclosures
Promissory note payable		$ 14,300
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
Letters of credit amount outstanding			39,600			39,600
Credit available			$ 219,400			$ 219,400
Principal financial covenants
Leverage ratio			0.50			0.50
Fixed charge coverage ratio			29.5			29.5
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
Other disclosures
Outstanding balance			$ 66,000			$ 66,000
Weighted average interest rate (as a percent)			2.60%			2.60%
Revolving credit facility | Through September 30, 2017
Principal financial covenants
Leverage ratio			3.00			3.00
Revolving credit facility | Through maturity
Principal financial covenants
Leverage ratio			2.75			2.75
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			1.50			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
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			1.50			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			$ 15,330			$ 15,330	2,250
Number of acquisitions | item	2
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			$ 239			$ 239	$ 305
Capital lease obligation pertaining to acquisition			100			100
Current portion of capital lease obligations			$ 100			$ 100