Long-Term Debt Obligations (Details 2)	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) item	Mar. 31, 2015 item	Sep. 30, 2015 USD ($) item	Sep. 30, 2014 USD ($)	Dec. 31, 2014 USD ($)
Reconciliation of Earnings before Interest Tax Depreciation and Amortization to Net Income
Goodwill impairment				$ 727,000
Additional per annum interest margin added under:
Payments of debt				$ 2,000,000
Other disclosures
Current portion of capital lease obligations	$ 360,000		$ 360,000		$ 317,000
Other Acquisitions
Other disclosures
Number of Businesses Acquired | item	1	2	3
Revolving credit facility
Long-Term Debt Obligations
Borrowing capacity	$ 250,000,000		$ 250,000,000
Outstanding borrowings	10,000,000		10,000,000
Letters of credit amount outstanding	46,400,000		46,400,000
Credit available	$ 193,600,000		$ 193,600,000
Principal financial covenants
Leverage ratio	0.10		0.10
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
Weighted average interest rate (as a percent)	1.50%		1.50%
Revolving credit facility | Environmental Air Systems, LLC
Long-Term Debt Obligations
Borrowing capacity	$ 2,500,000		$ 2,500,000
Outstanding borrowings	0		0
Credit available	$ 2,500,000		$ 2,500,000
Other disclosures
Weighted average interest rate (as a percent)	2.00%		2.00%
Revolving credit facility | Actual
Principal financial covenants
Fixed charge coverage ratio	30.60		30.60
Revolving credit facility | Minimum
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	2.0		2.0
Revolving credit facility | Minimum | Actual
Principal financial covenants
Fixed charge coverage ratio	2.0		2.0
Revolving credit facility | Maximum
Principal financial covenants
Permitted amount of acquisitions per transaction	$ 25,000,000		$ 25,000,000
Aggregate purchase price of current acquisition and acquisitions in the preceding 12 month period for determining permitted amount of acquisition per transaction	$ 60,000,000		$ 60,000,000
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 | Through maturity
Principal financial covenants
Leverage ratio	2.75		2.75
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	1.5		1.5
Revolving credit facility | Maximum | Actual
Principal financial covenants
Amount of stock repurchases to maintain maximum net leverage ratio			$ 25,000,000
Net leverage ratio after giving effect to stock repurchases for calculation of the fixed charge coverage ratio	1.5		1.5
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
Long-Term Debt Obligations
Outstanding balance	$ 1,000,000		$ 1,000,000
Other disclosures
Weighted average interest rate (as a percent)	2.50%		2.50%
Other debt | Dyna Ten
Other disclosures
Capital lease obligation pertaining to acquisition in northern Texas	$ 700,000		$ 700,000
Current portion of capital lease obligations	$ 400,000		$ 400,000