Long-Term Debt Obligations (Details 3) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Sep. 30, 2014 USD ($)	Jun. 30, 2014 USD ($)	Mar. 31, 2014 USD ($)	Dec. 31, 2015 USD ($) item	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($) item
Reconciliation of Earnings before Interest Tax Depreciation and Amortization to Net Income
Net income including noncontrolling interests	$ 14,883	$ 19,886	$ 15,782	$ 6,889	$ 11,821	$ 9,379	$ 6,336	$ 1,063	$ 57,440	$ 28,599	$ 28,556
Income taxes-continuing operations									(31,224)	(11,614)	$ (18,148)
Goodwill impairment										(727)
Other disclosures
Capital lease obligation pertaining to acquisition	500								500
Current portion of capital lease obligations	251				317				251	317
Number of acquisitions | item											0
Revolving credit facility
Long-Term Debt Obligations
Borrowing capacity	250,000								250,000
Financing and professional cost					600					600
Unamortized costs					$ 1,300					$ 1,300
Outstanding borrowings	10,000								10,000
Letters of credit amount outstanding	41,500								41,500
Credit available	198,500								198,500
Book value of assets pledged as collateral	$ 40,500								$ 40,500
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									$ 57,440
Income taxes-continuing operations									31,224
Interest expense, net									1,681
Depreciation and amortization expense									23,416
Stock-based compensation expense									5,609
EBITDA attributable to noncontrolling interests									(9,027)
Pre-acquisition results of acquired companies, as defined under the Facility									373
Credit Facility Adjusted EBITDA									$ 110,716
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
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)	1.70%								1.70%
Revolving credit facility | Environmental Air Systems, LLC
Long-Term Debt Obligations
Borrowing capacity	$ 2,500								$ 2,500
Outstanding borrowings	0								0
Credit available	$ 2,500								$ 2,500
Other disclosures
Weighted average interest rate (as a percent)	2.20%								2.20%
Revolving credit facility | Actual
Principal financial covenants
Fixed charge coverage ratio	27.45								27.45
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 | Covenant Requirement
Principal financial covenants
Net leverage ratio used as basis for other restrictions	2.00								2.00
Revolving credit facility | Minimum | Actual
Principal financial covenants
Fixed charge coverage ratio	2.00								2.00
Revolving credit facility | Maximum
Principal financial covenants
Permitted amount of acquisitions per transaction	$ 25,000								$ 25,000
Aggregate purchase price of current acquisition and acquisitions in the preceding 12 month period for determining permitted amount of acquisition per transaction	$ 60,000								$ 60,000
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	1.50								1.50
Revolving credit facility | Maximum | Actual
Principal financial covenants
Amount of stock repurchases to maintain maximum net leverage ratio									$ 25,000
Net leverage ratio after giving effect to stock repurchases for calculation of the fixed charge coverage ratio	1.50								1.50
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 | Base rate | Federal Funds Rate
Market rates relating to interest options
Interest rate margin (as a percent)									0.50%
Market rate (as a percent)	0.87%								0.87%
Additional per annum interest margin added under:
Additional per annum interest margin (as a percent)									0.50%
Revolving credit facility | Base rate | Wells Fargo Bank, N.A. Prime Rate
Market rates relating to interest options
Market rate (as a percent)	3.50%								3.50%
Revolving credit facility | Base rate | One-month LIBOR
Market rates relating to interest options
Interest rate margin (as a percent)									1.00%
Market rate (as a percent)	1.43%								1.43%
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%
Revolving credit facility | Eurodollar rate | One-month LIBOR
Market rates relating to interest options
Variable rate basis									One-month LIBOR
Market rate (as a percent)	0.43%								0.43%
Revolving credit facility | Eurodollar rate | Six-month LIBOR
Market rates relating to interest options
Variable rate basis									Six-month LIBOR
Market rate (as a percent)	0.86%								0.86%
Notes to former owners
Long-Term Debt Obligations
Outstanding borrowings	$ 1,000								$ 1,000
Other disclosures
Weighted average interest rate (as a percent)	2.50%								2.50%
Other debt
Other disclosures
Capital lease obligation pertaining to acquisition	$ 500								$ 500
Current portion of capital lease obligations	$ 300								$ 300
Number of acquisitions | item									1