Comfort Systems USA (FIX) 10-Q 27 Oct 21 2021 Q3 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	9 Months Ended
	Sep. 30, 2021	Oct. 22, 2021
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2021
Document Transition Report	false
Entity File Number	1-13011
Entity Registrant Name	COMFORT SYSTEMS USA, INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	76-0526487
Entity Address, Address Line One	675 Bering Drive
Entity Address, Address Line Two	Suite 400
Entity Address, City or Town	Houston
Entity Address, State or Province	TX
Entity Address, Postal Zip Code	77057
City Area Code	713
Local Phone Number	830-9600
Title of 12(b) Security	Common Stock, $0.01 par value
Trading Symbol	FIX
Security Exchange Name	NYSE
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		36,041,288
Entity Central Index Key	0001035983
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
Current Fiscal Year End Date	--12-31
Amendment Flag	false

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
CURRENT ASSETS:
Cash and cash equivalents	$ 67,721	$ 54,896
Billed accounts receivable, less allowance for credit losses of $7,618 and $9,087, respectively	700,629	619,544
Unbilled accounts receivable, less allowance for credit losses of $815 and $784, respectively	59,473	45,596
Other receivables, less allowance for credit losses of $503 and $759, respectively	30,489	44,212
Inventories	19,908	13,472
Prepaid expenses and other	11,519	15,510
Costs and estimated earnings in excess of billings, less allowance for credit losses of $34 and $79, respectively	14,972	18,622
Total current assets	904,711	811,852
PROPERTY AND EQUIPMENT, NET	115,538	117,206
LEASE RIGHT-OF-USE ASSET	104,336	94,727
GOODWILL	526,964	464,392
IDENTIFIABLE INTANGIBLE ASSETS, NET	268,099	231,807
DEFERRED TAX ASSETS	22,812	29,401
OTHER NONCURRENT ASSETS	8,874	7,970
Total assets	1,951,334	1,757,355
CURRENT LIABILITIES:
Current maturities of long-term debt	5,958
Accounts payable	236,387	204,145
Accrued compensation and benefits	137,910	121,864
Billings in excess of costs and estimated earnings	244,578	226,237
Accrued self-insurance	19,930	49,166
Other current liabilities	97,499	91,492
Total current liabilities	742,262	692,904
LONG-TERM DEBT, NET	266,772	235,733
LEASE LIABILITIES	88,923	80,576
DEFERRED TAX LIABILITIES	1,339	1,339
OTHER LONG-TERM LIABILITIES	80,249	50,374
Total liabilities	1,179,545	1,060,926
COMMITMENTS AND CONTINGENCIES
STOCKHOLDERS' EQUITY:
Preferred stock, $.01 par, 5,000,000 shares authorized, none issued and outstanding
Common stock, $.01 par, 102,969,912 shares authorized, 41,123,365 and 41,123,365 shares issued, respectively	411	411
Treasury stock, at cost, 5,080,907 and 4,935,186 shares, respectively	(150,987)	(129,243)
Additional paid-in capital	326,493	322,451
Retained earnings	595,872	502,810
Total stockholders' equity	771,789	696,429
Total liabilities and stockholders' equity	$ 1,951,334	$ 1,757,355

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
CONSOLIDATED BALANCE SHEETS
Billed accounts receivable, allowance for credit losses (in dollars)	$ 7,618	$ 9,087
Unbilled accounts receivable, allowance for credit losses (in dollars)	815	784
Other receivables, allowance for credit losses (in dollars)	503	759
Costs and estimated earnings in excess of billings, allowance for credit losses (in dollars)	$ 34	$ 79
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	102,969,912	102,969,912
Common stock, shares issued	41,123,365	41,123,365
Treasury stock, shares	5,080,907	4,935,186

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
CONSOLIDATED STATEMENTS OF OPERATIONS
REVENUE	$ 833,896	$ 714,099	$ 2,217,552	$ 2,157,698
COST OF SERVICES	674,684	566,903	1,808,416	1,747,714
Gross profit	159,212	147,196	409,136	409,984
SELLING, GENERAL AND ADMINISTRATIVE EXPENSES	95,287	90,888	271,050	268,857
GAIN ON SALE OF ASSETS	(180)	(377)	(1,021)	(1,243)
Operating income	64,105	56,685	139,107	142,370
OTHER INCOME (EXPENSE):
Interest income	1	7	7	99
Interest expense	(1,586)	(1,733)	(4,443)	(6,904)
Changes in the fair value of contingent earn-out obligations	(1,244)	3,423	4,523	1,824
Other	20	(15)	112	10
Other income (expense)	(2,809)	1,682	199	(4,971)
INCOME BEFORE INCOME TAXES	61,296	58,367	139,306	137,399
PROVISION FOR INCOME TAXES	14,999	8,279	33,553	30,100
NET INCOME	$ 46,297	$ 50,088	$ 105,753	$ 107,299
INCOME PER SHARE:
Basic	$ 1.28	$ 1.37	$ 2.91	$ 2.93
Diluted	$ 1.27	$ 1.36	$ 2.90	$ 2.92
SHARES USED IN COMPUTING INCOME PER SHARE:
Basic	36,296	36,557	36,328	36,604
Diluted	36,434	36,750	36,500	36,797

CONSOLIDATED STATEMENTS OF STOC

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Cumulative Effect, Period of Adoption, Adjustment [Member]Retained Earnings (Deficit)	Cumulative Effect, Period of Adoption, Adjustment [Member]	Common Stock	Treasury Stock	Additional Paid-In Capital	Retained Earnings (Deficit)	Total
BALANCE at Dec. 31, 2019			$ 411	$ (103,960)	$ 320,168	$ 368,685	$ 585,304
BALANCE (in shares) at Dec. 31, 2019			41,123,365
BALANCE (in shares) at Dec. 31, 2019				(4,465,448)
Increase (Decrease) in Stockholders' Equity
Net income						17,716	17,716
Issuance of Stock:
Issuance of restricted stock & performance stock				$ 1,054	801		1,855
Issuance of restricted stock & performance stock (in shares)				43,902
Shares received in lieu of tax withholding payment on vested restricted stock				$ (622)			(622)
Shares received in lieu of tax withholding payment on vested restricted stock (in shares)				(14,722)
Stock-based compensation					2,134		2,134
Dividends						(3,844)	(3,844)
Share repurchase				$ (8,985)			(8,985)
Share repurchase (in shares)				(237,359)
BALANCE at Mar. 31, 2020	$ (515)	$ (515)	$ 411	$ (112,513)	323,103	382,042	593,043
BALANCE (in shares) at Mar. 31, 2020			41,123,365
BALANCE (in shares) at Mar. 31, 2020				(4,673,627)
BALANCE at Dec. 31, 2019			$ 411	$ (103,960)	320,168	368,685	585,304
BALANCE (in shares) at Dec. 31, 2019			41,123,365
BALANCE (in shares) at Dec. 31, 2019				(4,465,448)
Increase (Decrease) in Stockholders' Equity
Net income							107,299
BALANCE at Sep. 30, 2020			$ 411	$ (118,551)	322,304	463,962	668,126
BALANCE (in shares) at Sep. 30, 2020			41,123,365
BALANCE (in shares) at Sep. 30, 2020				(4,723,672)
BALANCE at Mar. 31, 2020	$ (515)	$ (515)	$ 411	$ (112,513)	323,103	382,042	593,043
BALANCE (in shares) at Mar. 31, 2020			41,123,365
BALANCE (in shares) at Mar. 31, 2020				(4,673,627)
Increase (Decrease) in Stockholders' Equity
Net income						39,495	39,495
Issuance of Stock:
Issuance of shares for options exercised				$ 836	(280)		556
Issuance of shares for options exercised (in shares)				34,562
Issuance of restricted stock & performance stock				$ 2,048	(2,048)
Issuance of restricted stock & performance stock (in shares)				84,987
Shares received in lieu of tax withholding payment on vested restricted stock				$ (454)			(454)
Shares received in lieu of tax withholding payment on vested restricted stock (in shares)				(13,002)
Stock-based compensation					1,644		1,644
Dividends						(3,830)	(3,830)
Share repurchase				$ (2,021)			(2,021)
Share repurchase (in shares)				(52,991)
BALANCE at Jun. 30, 2020			$ 411	$ (112,104)	322,419	417,707	628,433
BALANCE (in shares) at Jun. 30, 2020			41,123,365
BALANCE (in shares) at Jun. 30, 2020				(4,620,071)
Increase (Decrease) in Stockholders' Equity
Net income						50,088	50,088
Issuance of Stock:
Issuance of shares for options exercised				$ 1,324	(325)		999
Issuance of shares for options exercised (in shares)				54,207
Stock-based compensation					210		210
Dividends						(3,833)	(3,833)
Share repurchase				$ (7,771)			(7,771)
Share repurchase (in shares)				(157,808)
BALANCE at Sep. 30, 2020			$ 411	$ (118,551)	322,304	463,962	668,126
BALANCE (in shares) at Sep. 30, 2020			41,123,365
BALANCE (in shares) at Sep. 30, 2020				(4,723,672)
BALANCE at Dec. 31, 2020			$ 411	$ (129,243)	322,451	502,810	$ 696,429
BALANCE (in shares) at Dec. 31, 2020			41,123,365				41,123,365
BALANCE (in shares) at Dec. 31, 2020				(4,935,186)			4,935,186
Increase (Decrease) in Stockholders' Equity
Net income						26,491	$ 26,491
Issuance of Stock:
Issuance of shares for options exercised				$ 1,616	(211)		1,405
Issuance of shares for options exercised (in shares)				61,454
Issuance of restricted stock & performance stock				$ 777	1,431		2,208
Issuance of restricted stock & performance stock (in shares)				29,544
Shares received in lieu of tax withholding payment on vested restricted stock				$ (854)			(854)
Shares received in lieu of tax withholding payment on vested restricted stock (in shares)				(11,424)
Stock-based compensation					2,472		2,472
Dividends						(4,163)	(4,163)
Share repurchase				$ (885)			(885)
Share repurchase (in shares)				(13,250)
BALANCE at Mar. 31, 2021			$ 411	$ (128,589)	326,143	525,138	723,103
BALANCE (in shares) at Mar. 31, 2021			41,123,365
BALANCE (in shares) at Mar. 31, 2021				(4,868,862)
BALANCE at Dec. 31, 2020			$ 411	$ (129,243)	322,451	502,810	$ 696,429
BALANCE (in shares) at Dec. 31, 2020			41,123,365				41,123,365
BALANCE (in shares) at Dec. 31, 2020				(4,935,186)			4,935,186
Increase (Decrease) in Stockholders' Equity
Net income							$ 105,753
BALANCE at Sep. 30, 2021			$ 411	$ (150,987)	326,493	595,872	$ 771,789
BALANCE (in shares) at Sep. 30, 2021			41,123,365				41,123,365
BALANCE (in shares) at Sep. 30, 2021				(5,080,907)			5,080,907
BALANCE at Mar. 31, 2021			$ 411	$ (128,589)	326,143	525,138	$ 723,103
BALANCE (in shares) at Mar. 31, 2021			41,123,365
BALANCE (in shares) at Mar. 31, 2021				(4,868,862)
Increase (Decrease) in Stockholders' Equity
Net income						32,965	32,965
Issuance of Stock:
Issuance of shares for options exercised				$ 1,853	191		2,044
Issuance of shares for options exercised (in shares)				69,342
Issuance of restricted stock & performance stock				$ 1,904	(1,904)
Issuance of restricted stock & performance stock (in shares)				71,816
Shares received in lieu of tax withholding payment on vested restricted stock				$ (1,509)			(1,509)
Shares received in lieu of tax withholding payment on vested restricted stock (in shares)				(19,989)
Stock-based compensation					1,749		1,749
Dividends						(4,178)	(4,178)
Share repurchase				$ (2,162)			(2,162)
Share repurchase (in shares)				(27,092)
BALANCE at Jun. 30, 2021			$ 411	$ (128,503)	326,179	553,925	752,012
BALANCE (in shares) at Jun. 30, 2021			41,123,365
BALANCE (in shares) at Jun. 30, 2021				(4,774,785)
Increase (Decrease) in Stockholders' Equity
Net income						46,297	46,297
Issuance of Stock:
Stock-based compensation					314		314
Dividends						(4,350)	(4,350)
Share repurchase				$ (22,484)			(22,484)
Share repurchase (in shares)				(306,122)
BALANCE at Sep. 30, 2021			$ 411	$ (150,987)	$ 326,493	$ 595,872	$ 771,789
BALANCE (in shares) at Sep. 30, 2021			41,123,365				41,123,365
BALANCE (in shares) at Sep. 30, 2021				(5,080,907)			5,080,907

CONSOLIDATED STATEMENTS OF ST_2

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Parenthetical) - $ / shares	3 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020
CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY
Dividends (per share)	$ 0.12	$ 0.115	$ 0.115	$ 0.105	$ 0.105	$ 0.105

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 105,753	$ 107,299
Adjustments to reconcile net income to net cash provided by operating activities-
Amortization of identifiable intangible assets	28,934	25,306
Depreciation expense	21,066	20,818
Change in right-of-use assets	12,904	12,338
Bad debt expense (benefit)	(1,698)	5,318
Deferred tax provision	6,589	1,197
Amortization of debt financing costs	403	408
Gain on sale of assets	(1,021)	(1,243)
Changes in the fair value of contingent earn-out obligations	(4,523)	(1,824)
Stock-based compensation	7,378	5,949
(Increase) decrease in-
Receivables, net	(25,809)	33,304
Inventories	(6,248)	(708)
Prepaid expenses and other current assets	15,592	9,417
Costs and estimated earnings in excess of billings and unbilled accounts receivable	(8,698)	(7,007)
Other noncurrent assets	(1,248)	(631)
Increase (decrease) in-
Accounts payable and accrued liabilities	15,112	(16,030)
Billings in excess of costs and estimated earnings	257	13,489
Other long-term liabilities	(12,089)	9,000
Net cash provided by operating activities	152,654	216,400
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of property and equipment	(15,864)	(19,459)
Proceeds from sales of property and equipment	1,802	1,890
Cash paid for acquisitions, net of cash acquired	(105,543)	(112,945)
Net cash used in investing activities	(119,605)	(130,514)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from revolving credit facility	160,000	198,000
Payments on revolving credit facility	(115,000)	(164,500)
Payments on term loan	(15,000)	(15,000)
Payments on other debt	(9,000)	(44,464)
Payments on finance lease liabilities	(207)
Payments of dividends to stockholders	(12,691)	(11,507)
Share repurchase	(25,531)	(18,777)
Shares received in lieu of tax withholding	(2,363)	(1,076)
Proceeds from exercise of options	3,449	1,555
Deferred acquisition payments	(400)	(500)
Payments for contingent consideration arrangements	(3,481)	(9,865)
Net cash used in financing activities	(20,224)	(66,134)
NET INCREASE IN CASH AND CASH EQUIVALENTS	12,825	19,752
CASH AND CASH EQUIVALENTS, beginning of period	54,896	50,788
CASH AND CASH EQUIVALENTS, end of period	$ 67,721	$ 70,540

Business and Organization

Business and Organization	9 Months Ended
	Sep. 30, 2021
Business and Organization
Business and Organization	1. Business and Organization ​ Comfort Systems USA, Inc., a Delaware corporation, provides comprehensive mechanical and electrical contracting services, which principally includes heating, ventilation and air conditioning (“HVAC”), plumbing, electrical, piping and controls, as well as off-site construction, monitoring and fire protection. We install, maintain, repair and replace products and systems throughout the United States. The terms “Comfort Systems,” “we,” “us,” or the “Company,” refer to Comfort Systems USA, Inc. or Comfort Systems USA, Inc. and its consolidated subsidiaries, as appropriate in the context. ​

Summary of Significant Accounti

Summary of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2021
Summary of Significant Accounting Policies
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies ​ Basis of Presentation ​ These interim statements should be read in conjunction with the historical Consolidated Financial Statements and related notes of Comfort Systems included in the Annual Report on Form 10-K as filed with the Securities and Exchange Commission (“SEC”) for the year ended December 31, 2020 (the “Form 10-K”). ​ The accompanying unaudited consolidated financial statements were prepared using generally accepted accounting principles for interim financial information and the instructions to Form 10-Q and applicable rules of Regulation S-X of the SEC. Accordingly, these financial statements do not include all the footnotes required by generally accepted accounting principles for complete financial statements and should be read in conjunction with the Form 10-K. We believe all adjustments necessary for a fair presentation of these interim statements have been included and are of a normal and recurring nature. The results of operations for interim periods are not necessarily indicative of the results for the full fiscal year. ​ Use of Estimates ​ The preparation of financial statements in conformity with generally accepted accounting principles requires the use of estimates and assumptions by management in determining the reported amounts of assets and liabilities, revenue and expenses and disclosures regarding contingent assets and liabilities. Actual results could differ from those estimates. The most significant estimates used in our financial statements affect revenue and cost recognition for construction contracts, self-insurance accruals, deferred tax assets, fair value accounting for acquisitions and the quantification of fair value for reporting units in connection with our goodwill impairment testing. ​ Recent Accounting Pronouncements ​ In December 2019, the FASB issued ASU No. 2019-12, “Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes.” This standard simplifies the accounting for income taxes by eliminating certain exceptions to the guidance in Topic 740 related to the approach for intraperiod tax allocation, the methodology for calculating income taxes in an interim period and the recognition of deferred tax liabilities for outside basis differences. The standard also simplifies aspects of the accounting for franchise taxes and enacted changes in tax laws or rates and clarifies the accounting for transactions that result in a step-up in the tax basis of goodwill. ASU No. 2019-12 is effective for fiscal years beginning after December 15, 2020 and interim periods within that year. We adopted ASU No. 2019-12 on January 1, 2021, and the impact was not material to our overall financial statements. ​ Revenue Recognition ​ We recognize revenue over time for all of our services as we perform them because (i) control continuously transfers to that customer as work progresses, and (ii) we have the right to bill the customer as costs are incurred. The customer typically controls the work in process, as evidenced either by contractual termination clauses or by our rights to payment for work performed to date, plus a reasonable profit, for delivery of products or services that do not have an alternative use to the Company. ​ For the reasons listed above, revenue is recognized based on the extent of progress towards completion of the performance obligation. The selection of the method to measure progress towards completion requires judgment and is based on the nature of the products or services to be provided. We generally use the cost to cost measure of progress for our contracts, as it best depicts the transfer of assets to the customer that occurs as we incur costs on our contracts. Under the cost to cost measure of progress, the extent of progress towards completion is measured based on the ratio of costs incurred to date to the total estimated costs at completion of the performance obligation. Revenue, including estimated fees or profits, is recorded proportionally as costs are incurred. Costs to fulfill include labor, materials, subcontractors’ costs, other direct costs and an allocation of indirect costs. ​ For a small portion of our business in which our services are delivered in the form of service maintenance agreements for existing systems to be repaired and maintained, as opposed to constructed, our performance obligation is to maintain the customer’s mechanical system for a specific period of time. Similar to construction jobs, we recognize revenue over time; however, for service maintenance agreements in which the full cost to provide services may not be known, we generally use an input method to recognize revenue, which is based on the amount of time we have provided our services out of the total time we have been contracted to perform those services. Our revenue recognition policy is further discussed in Note 3 “Revenue from Contracts with Customers.” ​ Accounts Receivable and Allowance for Credit Losses ​ We are required to estimate and record the expected credit losses over the contractual life of our financial assets measured at amortized cost, including billed and unbilled accounts receivable, other receivables and costs and estimated earnings in excess of billings. Accounts receivable include amounts from work completed in which we have billed or have an unconditional right to bill our customers. Our trade receivables are contractually due in less than a year. ​ We estimate our credit losses using a loss-rate method for each of our identified portfolio segments. Our portfolio segments are construction, service and other. While our construction and service financial assets are often with the same subset of customers and industries, our construction financial assets will generally have a lower loss-rate than service financial assets due to lien rights, which we are more likely to have on construction jobs. These lien rights result in lower credit loss expenses on average compared to receivables that do not have lien rights. Financial assets classified as Other include receivables that are not related to our core revenue producing activities, such as receivables related to our acquisition activity from former owners, our vendor rebate program or receivables for estimated losses in excess of our insurance deductible, which are accrued with a corresponding accrued insurance liability. ​ Loss rates for our portfolios are based on numerous factors, including our history of credit loss expense by portfolio, the financial strength of our customers and counterparties in each portfolio, the aging of our receivables, our expectation of likelihood of payment, macroeconomic trends in the U.S. and the current and forecasted non-residential construction market trends in the U.S. ​ In addition to the loss-rate calculations discussed above, we also record allowance for credit losses for specific receivables that are deemed to have a higher risk profile than the rest of the respective pool of receivables (e.g., when we hold concerns about a specific customer going bankrupt and no longer being able to pay the receivables due to us). ​ Income Taxes ​ We conduct business throughout the United States in virtually all fifty states. Our effective tax rate changes based upon our relative profitability, or lack thereof, in states with varying tax rates and rules. In addition, discrete items, such as tax law changes, judgments and legal structures, can impact our effective tax rate. These items can also include the tax treatment for impairment of goodwill and other intangible assets, changes in fair value of acquisition-related assets and liabilities, tax reserves for uncertain tax positions and accounting for losses associated with underperforming operations. ​ During the third quarter of 2020, the Internal Revenue Service (the “IRS”) completed its examination of our amended federal returns for 2014 and 2015 and issued a Revenue Agent Report allowing the $8.9 million of refund claims in full. Subsequently, the Joint Committee on Taxation reviewed and approved the refund claims. As a result, our provision for income taxes was reduced by $8.3 million due to a decrease in unrecognized tax benefits. ​ In early October 2020, we filed amended federal returns for 2016, 2017 and 2018 to claim the credit for increasing research activities (the “R&D tax credit”) and the energy efficient commercial buildings deduction (the “179D deduction”) allocated to us and recorded tax benefits of $6.1 million, $8.5 million and $11.9 million, respectively. The $26.5 million of tax benefits was offset by additions to unrecognized tax benefits of $26.4 million due to the uncertainty of the outcome of our current IRS examination. The R&D tax credit and 179D deduction for 2016, 2017 and 2018, therefore, had no material impact on our effective tax rates. ​ Financial Instruments ​ Our financial instruments consist of cash and cash equivalents, accounts receivable, other receivables, accounts payable, interest rate swaps, life insurance policies, notes to former owners, a revolving credit facility and a term loan. We believe that the carrying values of these instruments in the accompanying Balance Sheets approximate their fair values.

Revenue from Contracts with Cus

Revenue from Contracts with Customers	9 Months Ended
	Sep. 30, 2021
Revenue from Contract with Customers
Revenue from Contracts with Customers	3. Revenue from Contracts with Customers ​ Revenue is recognized when control of the promised goods or services is transferred to our customers, in an amount that reflects the consideration to which we expect to be entitled in exchange for those goods or services. Sales-based taxes are excluded from revenue. ​ We provide mechanical and electrical contracting services. Our mechanical segment principally includes HVAC, plumbing, piping and controls, as well as off- site construction, monitoring and fire protection. Our electrical segment includes installation and servicing of electrical systems. We install, maintain, repair and replace products and systems throughout the United States. All of our revenue is recognized over time as we deliver goods and services to our customers. Revenue can be earned based on an agreed-upon fixed price or based on actual costs incurred, marked up at an agreed-upon percentage. ​ We account for a contract when: (i) it has approval and commitment from both parties, (ii) the rights of the parties are identified, (iii) payment terms are identified, (iv) the contract has commercial substance, and (v) collectability of consideration is probable. We consider the start of a project to be when the above criteria have been met and we either have written authorization from the customer to proceed or an executed contract. ​ We generally do not incur significant incremental costs related to obtaining or fulfilling a contract prior to the start of a project. On rare occasions, when significant pre-contract costs are incurred, they are capitalized and amortized on a percentage of completion basis over the life of the contract. We do not currently have any capitalized obtainment or fulfillment costs on our Balance Sheet and have not incurred any impairment loss on such costs in the current year. ​ Due to the nature of the work required to be performed on many of our performance obligations, the estimation of total revenue and cost at completion (the process described below in more detail) is complex, subject to many variables and requires significant judgment. The consideration to which we are entitled on our long-term contracts may include both fixed and variable amounts. Variable amounts can either increase or decrease the transaction price. A common example of variable amounts that can either increase or decrease contract value are pending change orders that represent contract modifications for which a change in scope has been authorized or acknowledged by our customer, but the final adjustment to contract price is yet to be negotiated. Other examples of positive variable revenue include amounts awarded upon achievement of certain performance metrics, program milestones or cost of completion date targets and can be based upon customer discretion. Variable amounts can result in a deduction from contract revenue if we fail to meet stated performance requirements, such as complying with the construction schedule. ​ Contracts are often modified to account for changes in contract specifications and requirements. We consider contract modifications to exist when the modification either creates new, or changes the existing, enforceable rights and obligations. Most of our contract modifications are for goods or services that are not distinct from the existing performance obligation(s). The effect of a contract modification on the transaction price, and our measure of progress for the performance obligation to which it relates, is recognized as an adjustment to revenue (either as an increase or decrease) on a cumulative catchup basis. ​ We have a Company-wide policy requiring periodic review of the Estimate at Completion in which management reviews the progress and execution of our performance obligations and estimated remaining obligations. As part of this process, management reviews information including, but not limited to, any outstanding key contract matters, progress towards completion and the related program schedule, identified risks and opportunities and the related changes in estimates of revenue and costs. The risks and opportunities include management's judgment about the ability and cost to achieve the schedule (e.g., the number and type of milestone events), technical requirements (e.g., a newly developed product versus a mature product) and other contract requirements. Management must make assumptions and estimates regarding labor productivity and availability, the complexity of the work to be performed, the availability of materials, the length of time to complete the performance obligation (e.g., to estimate increases in wages and prices for materials and related support cost allocations), execution by our subcontractors, the availability and timing of funding from our customer, and overhead cost rates, among other variables. ​ Based on this analysis, any adjustments to revenue, cost of services, and the related impact to operating income are recognized as necessary in the quarter when they become known. These adjustments may result from positive program performance if we determine we will be successful in mitigating risks surrounding the technical, schedule and cost aspects of those performance obligations or realizing related opportunities and may result in an increase in operating income during the performance of individual performance obligations. Likewise, if we determine we will not be successful in mitigating these risks or realizing related opportunities, these adjustments may result in a decrease in operating income. Changes in estimates of revenue, cost of services and the related impact to operating income are recognized quarterly on a cumulative catchup basis, meaning we recognize in the current period the cumulative effect of the changes on current and prior periods based on a performance obligation's percentage of completion. A significant change in one or more of these estimates could affect the profitability of one or more of our performance obligations. For projects in which estimates of total costs to be incurred on a performance obligation exceed total estimates of revenue to be earned, a provision for the entire loss on the performance obligation is recognized in the period the loss is determined. ​ In the first nine months of 2021 and 2020, net revenue recognized from our performance obligations satisfied in previous periods was not material. ​ Disaggregation of Revenue ​ Our consolidated 2021 revenue was derived from contracts to provide service activities in the mechanical and electrical services segments we serve. Refer to Note 11 “Segment Information” for additional information on our reportable segments. We disaggregate our revenue from contracts with customers by activity, customer type and service provided, as we believe it best depicts how the nature, amount, timing and uncertainty of our revenue and cash flows are affected by economic factors. See details in the following tables (dollars in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ ​ Nine Months Ended September 30, Revenue by Service Provided 2021 ​ 2020 ​ 2021 ​ 2020 ​ Mechanical Services ​ $ 690,680 82.8 % ​ $ 625,660 87.6 % ​ $ 1,868,096 84.2 % ​ $ 1,823,117 84.5 % Electrical Services ​ ​ 143,216 ​ 17.2 % ​ ​ 88,439 ​ 12.4 % ​ ​ 349,456 ​ 15.8 % ​ ​ 334,581 ​ 15.5 % Total ​ $ 833,896 ​ 100.0 % ​ $ 714,099 ​ 100.0 % ​ $ 2,217,552 ​ 100.0 % ​ $ 2,157,698 ​ 100.0 % ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ ​ Nine Months Ended September 30, Revenue by Type of Customer ​ 2021 ​ ​ 2020 ​ 2021 ​ ​ 2020 Industrial ​ $ 379,605 ​ 45.5 % ​ $ 268,651 ​ 37.6 % ​ $ 960,263 ​ 43.3 % ​ $ 844,719 ​ 39.1 % Education ​ ​ 115,536 ​ 13.9 % ​ ​ 135,992 ​ 19.0 % ​ ​ 300,374 ​ 13.5 % ​ ​ 375,580 ​ 17.4 % Office Buildings ​ ​ 77,806 ​ 9.3 % ​ ​ 81,183 ​ 11.4 % ​ ​ 229,816 ​ 10.4 % ​ ​ 232,349 ​ 10.8 % Healthcare ​ ​ 101,446 ​ 12.2 % ​ ​ 86,061 ​ 12.1 % ​ ​ 292,541 ​ 13.2 % ​ ​ 281,370 ​ 13.0 % Government ​ ​ 40,268 ​ 4.8 % ​ ​ 40,631 ​ 5.7 % ​ ​ 125,939 ​ 5.7 % ​ ​ 119,444 ​ 5.5 % Retail, Restaurants and Entertainment ​ ​ 62,365 ​ 7.5 % ​ ​ 64,212 ​ 9.0 % ​ ​ 155,874 ​ 7.0 % ​ ​ 190,043 ​ 8.8 % Multi-Family and Residential ​ ​ 29,741 ​ 3.6 % ​ ​ 21,776 ​ 3.0 % ​ ​ 82,742 ​ 3.7 % ​ ​ 61,062 ​ 2.8 % Other ​ ​ 27,129 ​ 3.2 % ​ ​ 15,593 ​ 2.2 % ​ ​ 70,003 ​ 3.2 % ​ ​ 53,131 ​ 2.6 % Total ​ $ 833,896 ​ 100.0 % ​ $ 714,099 ​ 100.0 % ​ $ 2,217,552 ​ 100.0 % ​ $ 2,157,698 ​ 100.0 % ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ ​ Nine Months Ended September 30, Revenue by Activity Type ​ 2021 ​ ​ 2020 ​ 2021 ​ ​ 2020 New Construction ​ $ 397,446 ​ 47.6 % ​ $ 310,000 ​ 43.4 % ​ $ 1,029,397 ​ 46.4 % ​ $ 1,034,833 ​ 48.0 % Existing Building Construction ​ ​ 250,893 ​ 30.1 % ​ ​ 235,798 ​ 33.0 % ​ ​ 682,811 ​ 30.8 % ​ ​ 668,067 ​ 30.9 % Service Projects ​ ​ 75,525 ​ 9.1 % ​ ​ 69,162 ​ 9.7 % ​ ​ 201,848 ​ 9.1 % ​ ​ 179,188 ​ 8.3 % Service Calls, Maintenance and Monitoring ​ ​ 110,032 ​ 13.2 % ​ ​ 99,139 ​ 13.9 % ​ ​ 303,496 ​ 13.7 % ​ ​ 275,610 ​ 12.8 % Total ​ $ 833,896 ​ 100.0 % ​ $ 714,099 ​ 100.0 % ​ $ 2,217,552 ​ 100.0 % ​ $ 2,157,698 ​ 100.0 % ​ Contract Assets and Liabilities ​ Project contracts typically provide for a schedule of billings or invoices to the customer based on our job-to-date percentage of completion of specific tasks inherent in the fulfillment of our performance obligation(s). The schedules for such billings usually do not precisely match the schedule on which costs are incurred. Contract assets include unbilled amounts typically resulting from sales under long term contracts when the cost to cost method of revenue recognition is used, revenue recognized exceeds the amount billed to the customer and right to payment is conditional or subject to completing a milestone, such as a phase of the project. Contract assets are generally classified as current. ​ Contract liabilities consist of advance payments and billings in excess of revenue recognized. Our contract assets and liabilities are reported in a net position on a contract by contract basis at the end of each reporting period. We classify advance payments and billings in excess of revenue recognized as current. It is very unusual for us to have advanced payments with a term of greater than one year; therefore, our contract assets and liabilities are usually all current. If we have advanced payments with a term greater than one year, the noncurrent portion of advanced payments would be included in other long-term liabilities in our consolidated Balance Sheets. ​ The following table presents the changes in contract assets and contract liabilities (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Nine Months Ended September 30, ​ ​ Year Ended December 31, ​ 2021 ​ 2020 ​ Contract Contract ​ Contract Contract ​ Assets ​ Liabilities ​ Assets ​ Liabilities Balance at beginning of period $ 18,622 ​ $ 226,237 ​ $ 2,736 ​ $ 166,918 Change due to acquisitions / disposals ​ 2,378 ​ ​ 18,084 ​ ​ 9,509 ​ ​ 39,885 Change related to credit allowance ​ 45 ​ ​ — ​ ​ (79) ​ ​ — Other changes in the period ​ (6,073) ​ ​ 257 ​ ​ 6,456 ​ ​ 19,434 Balance at end of period $ 14,972 ​ $ 244,578 ​ $ 18,622 $ 226,237 ​ In the first nine months of 2021 and 2020, we recognized revenue of $204.6 million and $164.4 million related to our contract liabilities at January 1, 2021 and January 1, 2020, respectively. ​ We did not have any impairment losses recognized on our receivables or contract assets in the first nine months of 2021 and 2020. ​ Remaining Performance Obligations ​ Remaining construction performance obligations represent the remaining transaction price of firm orders for which work has not been performed and exclude unexercised contract options. As of September 30, 2021, the aggregate amount of the transaction price allocated to remaining performance obligations was $1.94 billion. The Company expects to recognize revenue on approximately 80-85% of the remaining performance obligations over the next 12 months , with the remaining recognized thereafter. Our service maintenance agreements are generally one-year renewable agreements. We have adopted the practical expedient that allows us to not include service maintenance contracts with a total term of one year or less ; therefore, we do not report unfulfilled performance obligations for service maintenance agreements.

Fair Value Measurements

Fair Value Measurements	9 Months Ended
	Sep. 30, 2021
Fair Value Measurements
Fair Value Measurements	4. Fair Value Measurements ​ Interest Rate Risk Management and Derivative Instruments In April 2020, we entered into interest rate swap agreements to reduce our exposure to variable interest rates on our term loan and revolving credit facility. The notional amount covered by these interest rate swaps was $130.0 million as of September 30, 2021 and decreases to $80.0 million by November 30, 2021 until the termination date of September 30, 2022. ​ We use derivative instruments to manage exposure to market risk, including interest rate risk. Unsettled amounts under our interest rate swaps are recorded on the Balance Sheet at fair value in “Other Receivables” or “Other Current Liabilities.” Gains and losses on our interest rate swaps are recorded on the Income Statement in “Interest Expense.” For the three months ended September 30, 2021 and September 30, 2020, we recognized a net loss of $0.1 million and $0.2 million, respectively, related to our interest rate swaps. For the nine months ended September 30, 2021 and September 30, 2020, we recognized a net loss of $0.4 million and $0.2 million, respectively, related to our interest rate swaps. We currently do not have any derivatives that are accounted for as hedges under ASC 815. ​ Fair Value Measurements ​ We classify and disclose assets and liabilities carried at fair value in one of the following three categories: ​ ● Level 1—quoted prices in active markets for identical assets and liabilities; ● Level 2—observable market-based inputs or unobservable inputs that are corroborated by market data; and ● Level 3—significant unobservable inputs in which little or no market data exists, therefore requiring an entity to develop its own assumptions. ​ The following table summarizes the fair values, and levels within the fair value hierarchy in which the fair value measurements fall, for assets and liabilities measured on a recurring basis as of September 30, 2021 and December 31, 2020 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at September 30, 2021 ​ Level 1 Level 2 Level 3 Total Cash and cash equivalents ​ $ 67,721 ​ $ — ​ $ — ​ $ 67,721 Life insurance—cash surrender value ​ $ — ​ $ 6,683 ​ $ — ​ $ 6,683 Contingent earn-out obligations ​ $ — ​ $ — ​ $ 33,780 ​ $ 33,780 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 2020 ​ Level 1 Level 2 Level 3 Total Cash and cash equivalents ​ $ 54,896 ​ $ — ​ $ — ​ $ 54,896 Life insurance—cash surrender value ​ $ — ​ $ 5,420 ​ $ — ​ $ 5,420 Contingent earn-out obligations ​ $ — ​ $ — ​ $ 25,979 ​ $ 25,979 Interest rate swap liability ​ $ — ​ $ 42 ​ $ — ​ $ 42 ​ Cash and cash equivalents consist primarily of highly rated money market funds at a variety of well-known institutions with original maturities of three months or less. The original cost of these assets approximates fair value due to their short-term maturity. The fair value for our interest rate swaps is based upon inputs corroborated by observable market data with similar tenors, which are considered Level 2 inputs. The Company’s outstanding term loan held by third-party financial institutions is carried at cost, adjusted for debt issuance costs. The Company’s term loan is not publicly traded and the carrying amount approximates fair value as the loan accrues interest at a variable rate. The carrying value of our borrowings associated with the revolving credit facility approximate its fair value due to the variable rate on such debt. ​ We have life insurance policies covering 89 employees with a combined face value of $63.9 million. The policies are invested in several investment vehicles, and the fair value measurement of the cash surrender balance associated with these policies is determined using Level 2 inputs within the fair value hierarchy and will vary with investment performance. The cash surrender value of these policies was $6.7 million as of September 30, 2021 and $5.4 million as of December 31, 2020. These assets are included in “Other Noncurrent Assets” in our consolidated Balance Sheets. ​ We value contingent earn-out obligations using a probability weighted discounted cash flow method. This fair value measurement is based on significant unobservable inputs in the market and thus represents a Level 3 measurement within the fair value hierarchy. This analysis reflects the contractual terms of the purchase agreements (e.g., minimum and maximum payments, length of earn-out periods, manner of calculating any amounts due, etc.) and utilizes assumptions with regard to future cash flows and operating income, probabilities of achieving such future cash flows and operating income and a weighted average cost of capital. Significant changes in any of these assumptions could result in a significantly higher or lower potential liability. The contingent earn-out obligations are measured at fair value each reporting period, and changes in estimates of fair value are recognized in earnings. As of September 30, 2021, cash flows were discounted using a weighted average cost of capital ranging from 10.5% - 17.0%. ​ The table below presents a reconciliation of the fair value of our contingent earn-out obligations that use significant unobservable inputs (Level 3) (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Nine Months Ended ​ Year Ended ​ ​ September 30, 2021 ​ December 31, 2020 ​ Balance at beginning of period $ 25,979 ​ $ 28,497 Issuances ​ 16,155 ​ 16,715 ​ Settlements ​ ​ (3,831) ​ ​ (10,114) ​ Adjustments to fair value ​ (4,523) ​ (9,119) ​ Balance at end of period ​ $ 33,780 ​ $ 25,979 ​ ​

Acquisitions

Acquisitions	9 Months Ended
	Sep. 30, 2021
Acquisitions
Acquisitions	5. Acquisitions ​ Amteck Holdco LLC Acquisition ​ On August 1, 2021, we acquired all of the issued and outstanding equity interests of Amteck Holdco LLC and each of its wholly owned subsidiaries (collectively “Amteck”). Amteck provides electrical contracting solutions and services, including design and build, pre-fabrication and installation for core electric and low-voltage systems, as well as services for planned maintenance, retrofit and emergency work. Amteck is headquartered in Kentucky and primarily serves the greater Southeastern United States, including Kentucky, Tennessee and the Carolinas. As a result of the acquisition, Amteck is a wholly owned subsidiary of the Company reported in our electrical services segment. ​ The following summarizes the acquisition date fair value of consideration transferred and the acquisition date fair value of the identifiable assets acquired and liabilities assumed, including an amount for goodwill (in thousands): ​ ​ ​ ​ Consideration transferred: ​ ​ Cash paid at closing $ 107,355 Working capital adjustment ​ 6,892 Notes issued to former owners ​ 10,000 Estimated fair value of contingent earn-out payments ​ 12,900 ​ $ 137,147 Recognized amounts of identifiable assets acquired and liabilities assumed: ​ ​ Cash and cash equivalents $ 13,554 Billed and unbilled accounts receivable ​ 43,248 Other current assets ​ 3,422 Property and equipment ​ 3,891 Goodwill ​ 54,554 Identifiable intangible assets ​ 57,700 Operating lease right-of-use asset ​ 8,997 Accounts payable ​ (14,489) Billings in excess of costs and estimated earnings ​ (15,199) Current maturities of long-term debt ​ (1,860) Current operating lease liabilities ​ (1,115) Accrued expenses and other current liabilities ​ (6,889) Long-term debt ​ (785) Long-term operating lease liabilities ​ (7,882) ​ $ 137,147 ​ The allocation of the purchase price to the assets acquired and liabilities assumed is preliminary and, therefore, subject to change pending the completion of the final valuation of intangible assets and accrued liabilities. Goodwill represents the future economic benefits arising from other assets acquired that could not be individually identified and separately recognized. All of the goodwill recognized as a result of the Amteck acquisition is tax deductible. ​ In estimating the fair value of the acquired intangible assets, we utilized the valuation methodology determined to be the most appropriate for the individual intangible asset. In order to estimate the fair value of the backlog and customer relationships, we utilized an excess earnings methodology, which consisted of the projected cash flows attributable to these assets discounted to present value using a risk-adjusted discount rate that represented the required rate of return. The trade name value was determined based on the relief-from-royalty method, which applies a royalty rate to the revenue stream attributable to this asset, and the resulting royalty payment is tax effected and discounted to present value. Some of the more significant estimates and assumptions inherent in determining the fair value of the identifiable intangible assets are associated with forecasting cash flows and profitability, which represent Level 3 inputs. The primary assumptions used were generally based upon the present value of anticipated cash flows discounted at rates ranging from 14.0%-18.5 %. Estimated years of projected earnings generally follow the range of estimated remaining useful lives for each intangible asset class. ​ The acquired intangible assets include the following (dollars in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Valuation Method ​ Estimated Useful Life ​ ​ Estimated Fair Value Backlog ​ Excess earnings ​ 1.5 years ​ $ 7,800 Trade Name ​ Relief-from-royalty ​ 23 years ​ ​ 12,600 Customer Relationships ​ Excess earnings ​ 10 years ​ ​ 37,300 Total ​ ​ ​ ​ ​ $ 57,700 ​ The contingent earn-out obligation is associated with the achievement of four earnings milestones over a 53-month period, and the range of each average present value estimated milestone payment is $0.1 million to $11.4 million. We determined the initial fair value of the contingent earn-out obligation based on the Monte Carlo Simulation method, which represents a Level 3 measurement. Cash flows were discounted using discount rates ranging from 14.8%-15.2%, which we believe is appropriate and representative of a market participant assumption. Subsequent to the acquisition date, the contingent earn-out obligation is remeasured at fair value each reporting period. Changes in the estimated fair value of the contingent payments subsequent to the acquisition date are recognized immediately in earnings. ​ Other Acquisitions ​ In the first quarter of 2021, we completed an acquisition of a mechanical contractor in Utah with a total preliminary purchase price of $17.9 million, which is reported in our mechanical services segment. In the fourth quarter of 2020, we acquired all outstanding equity interest of Tennessee Electric Company, Inc. dba TEC Industrial Maintenance and Construction (“TEC”) for a total preliminary purchase price of $88.9 million, which included $73.0 million in cash, $7.0 million in notes payable to former owners, a $7.6 million contingent earn-out obligation and a $1.3 million working capital adjustment. As a result of the acquisition, TEC is a wholly owned subsidiary of the Company reported in our electrical services segment. We completed the acquisition of an electrical contractor in North Carolina in the first quarter of 2020 with a total purchase price of $41.6 million. This acquisition is reported in our electrical services segment. ​ On April 1, 2020, we consummated a merger through which TAS Energy Inc. (“TAS”) became a wholly owned subsidiary of the Company. The total purchase price was $169.5 million, of which $126.2 million was allocated to goodwill and identifiable intangible assets. The total purchase price included $105.9 million in cash, $40.5 million in working capital adjustment, $14.0 million in notes payable to former owners and a $9.1 million contingent earn-out obligation. TAS is headquartered in Houston, Texas, and is a leading engineering, design and construction provider of modular construction systems serving the technology, power and industrial sectors, and reports as a separate operating location in our mechanical services segment. ​ The results of operations of acquisitions are included in our consolidated financial statements from their respective acquisition dates. Our consolidated Balance Sheet includes preliminary allocations of the purchase price to the assets acquired and liabilities assumed for the applicable acquisitions pending the completion of the final valuation of intangible assets and accrued liabilities. The acquisitions completed in the current and prior year were not material, individually or in the aggregate. Additional contingent purchase price (“earn-out”) has been or will be paid if certain acquisitions achieve predetermined profitability targets. Such earn-outs, when they are not subject to the continued employment of the sellers, are estimated as of the purchase date and included as part of the consideration paid for the acquisition. If we have an earn-out under which continued employment is a condition to receipt of payment, then the earn-out is recorded as compensation expense over the period earned.

Goodwill and Identifiable Intan

Goodwill and Identifiable Intangible Assets, Net	9 Months Ended
	Sep. 30, 2021
Goodwill and Identifiable Intangible Assets, Net
Goodwill and Identifiable Intangible Assets, Net	6. Goodwill and Identifiable Intangible Assets, Net ​ Goodwill ​ The changes in the carrying amount of goodwill are as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Mechanical Services ​ Electrical Services ​ ​ ​ Segment Segment ​ Total Balance at December 31, 2019 ​ $ 234,660 ​ $ 97,787 ​ $ 332,447 Acquisitions and purchase price adjustments (See Note 5) ​ 72,788 ​ ​ 59,157 ​ ​ 131,945 Balance at December 31, 2020 ​ ​ 307,448 ​ ​ 156,944 ​ ​ 464,392 Acquisitions and purchase price adjustments (See Note 5) ​ ​ 8,089 ​ ​ 54,483 ​ ​ 62,572 Impact of segment reorganization ​ ​ 1,101 ​ ​ (1,101) ​ ​ — Balance at September 30, 2021 ​ $ 316,638 ​ $ 210,326 ​ $ 526,964 ​ During the fourth quarter of 2020, the Company performed a qualitative assessment for all of our reporting units except one for which we performed a quantitative assessment, which considered various factors, including changes in the carrying value of the reporting unit, forecasted operating results, long-term growth rates and discount rates. Additionally, we considered qualitative key events and circumstances (i.e. macroeconomic environment, industry and market specific conditions, cost factors and events specific to the reporting unit, etc.). Based on this assessment, we concluded that it was more likely than not that the fair value of each of the reporting units was greater than its carrying value. Accordingly, no further testing was required. For our Texas electrical operation, we performed a step 1 quantitative assessment, and the calculated fair value exceeded the carrying value by 24%. As a result of uncertainty caused by COVID-19 and the reporting unit’s smaller excess of fair value percentage, this reporting unit is more susceptible to impairment risk from additional adverse changes in its operating environment, including micro- and macroeconomic environment conditions that could negatively impact them. Such adverse changes could include worsening economic conditions in the locations or markets they primarily serve, whether due to COVID-19 or other events and conditions. As of September 30, 2021, the Texas electrical operation had a goodwill balance of $96.8 million. Identifiable Intangible Assets, Net ​ At September 30, 2021, future amortization expense of identifiable intangible assets is as follows (in thousands): ​ ​ ​ ​ ​ ​ Year ended December 31— ​ ​ 2021 (remainder of the year) $ 11,021 2022 ​ ​ 36,968 ​ 2023 ​ 28,495 ​ 2024 ​ 27,145 ​ 2025 ​ 24,958 ​ Thereafter ​ 139,512 ​ Total ​ $ 268,099 ​ ​

Debt Obligations

Debt Obligations	9 Months Ended
	Sep. 30, 2021
Debt Obligations
Debt Obligations	7. Debt Obligations ​ Debt obligations consist of the following (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, ​ December 31, ​ ​ 2021 2020 Revolving credit facility ​ $ 115,000 ​ $ 70,000 ​ Term loan ​ ​ 120,000 ​ ​ 135,000 ​ Notes to former owners ​ ​ 35,500 ​ 31,000 ​ Finance lease liabilities (See Note 8) ​ ​ 2,438 ​ ​ — ​ Total principal amount ​ ​ 272,938 ​ 236,000 ​ Less—unamortized debt issuance costs ​ ​ (208) ​ ​ (267) ​ Total debt, net of unamortized debt issuance costs ​ ​ 272,730 ​ ​ 235,733 ​ Less—current portion ​ ​ (5,958) ​ — ​ Total long-term portion of debt, net ​ $ 266,772 ​ $ 235,733 ​ ​ Revolving Credit Facility and Term Loan ​ We have a $600.0 million senior credit facility (the “Facility”) provided by a syndicate of banks. The Facility is composed of a revolving credit line in the amount of $450.0 million and a $150.0 million term loan, and the Facility provides for a $150.0 million accordion or increase option for the revolving portion of the Facility. As of September 30, 2021, the Facility capacity was $570.0 million, as the term loan was paid down by $30.0 million since the inception of the Facility. The amended Facility also includes a sublimit of up to $160.0 million issuable in the form of letters of credit. The Facility expires in January 2025 and is secured by a first lien on substantially all of our personal property except for assets related to projects subject to surety bonds and assets held by certain unrestricted subsidiaries and our wholly owned captive insurance company, and a second lien on our assets related to projects subject to surety bonds. As of September 30, 2021, we had $115.0 million of outstanding borrowings on the revolving credit facility, $49.5 million in letters of credit outstanding and $285.5 million of credit available. ​ There are two interest rate options for borrowings under the Facility, the Base Rate Loan Option and the Eurodollar Rate Loan Option. These rates are floating rates determined by the broad financial markets, meaning they can and do move up and down from time to time. Additional margins are then added to these two rates. ​ The following is a summary of the additional margins: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Consolidated Total Indebtedness to ​ ​ Credit Facility Adjusted EBITDA ​ Less than 1.00 1.00 to 1.75 1.75 to 2.50 2.50 or greater Additional Per Annum Interest Margin Added Under: ​ ​ ​ ​ ​ ​ ​ ​ ​ Base Rate Loan Option ​ 0.25 % 0.50 % 0.75 % 1.00 % Eurodollar Rate Loan Option ​ 1.25 % 1.50 % 1.75 % 2.00 % ​ The weighted average interest rate applicable to the borrowings under the revolving credit facility was approximately 1.3% as of September 30, 2021. The weighted average interest rate applicable to the term loan was approximately 1.3% as of September 30, 2021. ​ Certain of our vendors require letters of credit to ensure reimbursement for amounts they are disbursing on our behalf, such as to beneficiaries under our self-funded insurance programs. We have also occasionally used letters of credit to guarantee performance under our contracts and to ensure payment to our subcontractors and vendors under those contracts. Our lenders issue such letters of credit through the Facility. We have never had a claim made against a letter of credit that resulted in payments by a lender or by us and believe such a claim is unlikely in the foreseeable future. The letter of credit fees range from 1.25% to 2.00% per annum, based on the ratio of Consolidated Total Indebtedness to “Credit Facility Adjusted EBITDA,” which shall mean Consolidated EBITDA as such term is defined in the credit agreement. ​ Commitment fees are payable on the portion of the revolving loan capacity not in use for borrowings or letters of credit at any given time. These fees range from 0.20% to 0.35% per annum, based on the ratio of Consolidated Total Indebtedness to Credit Facility Adjusted EBITDA. ​ The Facility contains financial covenants defining various financial measures and the levels of these measures with which we must comply. Covenant compliance is assessed as of each quarter end. We were in compliance with all of our financial covenants as of September 30, 2021. ​ Notes to Former Owners ​ As part of the consideration used to acquire six companies, we have outstanding notes to the former owners. Together, these notes had an outstanding balance of $35.5 million as of September 30, 2021. In conjunction with the acquisition of Amteck in the third quarter of 2021, we issued a promissory note to former owners with an outstanding balance of $10.0 million as of September 30, 2021 that bears interest, payable quarterly, at a stated interest rate of 2.5%. The principal is due in equal installments in October 2024 and October 2025. In conjunction with the acquisition of the Utah mechanical contractor in the first quarter of 2021, we issued a promissory note to former owners with an outstanding balance of $3.5 million as of September 30, 2021 that bears interest, payable quarterly, at a stated interest rate of 2.5%. The principal is due in April 2023. In conjunction with the acquisition of TEC in the fourth quarter of 2020, we issued a promissory note to former owners with an outstanding balance of $7.0 million as of September 30, 2021 that bears interest, payable quarterly, at a stated interest rate of 2.5 %. The principal is due in December 2023. In conjunction with the acquisition of TAS in the second quarter of 2020, we issued a promissory note to former owners with an outstanding balance of $4.0 million as of September 30, 2021 that bears interest, payable quarterly, at a stated interest rate of 3.5 %. The principal is due in April 2022. In conjunction with the acquisition of the electrical contractor in North Carolina in the first quarter of 2020, we issued a promissory note to former owners with an outstanding balance of $5.0 million as of September 30, 2021 that bears interest, payable quarterly, at a stated interest rate of 3.0%. The principal is due in installments in February 2023 and February 2024. In conjunction with the acquisition of a Texas electrical contractor in the second quarter of 2019, we issued a promissory note to former owners with an outstanding balance of $6.0 million as of September 30, 2021 that bears interest, payable quarterly, at a stated interest rate of 4.0 %. The remaining principal is due in April 2023.

Leases

Leases	9 Months Ended
	Sep. 30, 2021
Leases
Leases	8. Leases ​ We lease certain facilities, vehicles and equipment primarily under noncancelable operating leases. The most significant portion of these noncancelable operating leases are for the facilities occupied by our corporate office and our operating locations. As a result of our acquisition of Amteck in August 2021, we acquired some finance leases on vehicles that are not material to our consolidated financial position. As of September 30, 2021, the lease liabilities associated with these finance leases was $2.4 million. Leases with an initial term of 12 months or less are not recorded in the Balance Sheet. We do not separate lease components from their associated non-lease components pursuant to lease accounting guidance. We have certain leases with variable payments based on an index as well as some short-term leases on equipment and facilities. Variable lease expense and short-term lease expense were not material to our financial statements and aggregated to $5.7 million and $6.1 million in the first nine months of 2021 and 2020, respectively. Lease right-of-use assets and liabilities are recognized at commencement date based on the present value of lease payments over the lease term. As most of our leases do not provide an implicit rate, we generally use our incremental borrowing rate based on the information available at commencement date in determining the present value of lease payments. The weighted average discount rate for our operating leases as of September 30, 2021 and December 31, 2020 was 4.1% and 4.2%, respectively. We recognize operating lease expense, including escalating lease payments and lease incentives, on a straight-line basis over the lease term. Operating lease expense for the three months ended September 30, 2021 and 2020 was $7.7 million and $8.0 million, respectively. Operating lease expense for the nine months ended September 30, 2021 and 2020 was $22.1 million and $21.4 million, respectively. ​ The lease terms generally range from three ​ A majority of the Company’s real property leases are with individuals or entities with whom we have no other business relationship. However, in certain instances the Company enters into real property leases with current or former employees. Rent paid to related parties for the three months ended September 30, 2021 and 2020 was approximately $1.1 million and $2.0 million, respectively. Rent paid to related parties for the nine months ended September 30, 2021 and 2020 was approximately $3.6 million and $4.5 million, respectively. ​ If we decide to cancel or terminate a lease before the end of its term, we would typically owe the lessor the remaining lease payments under the term of the lease. Our lease agreements do not contain any material residual value guarantees or material restrictive covenants. On rare occasions, we rent or sublease certain real estate assets that we no longer use to third parties. ​ Finance lease right-of-use assets are included in “Property and equipment, net,” and current and long-term finance lease liabilities are included within “Current maturities of long-term debt” and “Long-term debt, net,” respectively, in the consolidated Balance Sheet. The following table summarizes the operating lease assets and liabilities included in the consolidated Balance Sheet as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ September 30, 2021 ​ December 31, 2020 Operating lease right-of-use assets $ 104,336 ​ $ 94,727 Operating lease liabilities: ​ ​ ​ ​ ​ Other current liabilities $ 17,499 ​ $ 16,586 Long-term operating lease liabilities ​ 88,923 ​ ​ 80,576 Total operating lease liabilities $ 106,422 ​ $ 97,162 ​ The maturities of operating lease liabilities are as follows (in thousands): ​ ​ ​ ​ ​ Year ending December 31— ​ ​ ​ 2021 (excluding the nine months ended September 30, 2021) ​ $ 5,668 2022 ​ ​ 20,771 2023 ​ ​ 18,101 2024 ​ ​ 16,066 2025 ​ ​ 15,180 Thereafter ​ ​ 49,092 Total Lease Payments ​ ​ 124,878 Less—Present Value Discount ​ ​ (18,456) Present Value of Operating Lease Liabilities ​ $ 106,422 ​ Supplemental information related to operating leases was as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ Nine Months Ended September 30, ​ 2021 ​ 2020 2021 ​ 2020 Cash paid for amounts included in the measurement of operating lease liabilities $ 5,597 ​ $ 5,362 ​ $ 16,297 ​ $ 15,140 Operating lease right-of-use assets obtained in exchange for lease liabilities $ 13,154 ​ $ 1,880 ​ $ 22,513 ​ $ 24,080 ​

Commitments and Contingencies

Commitments and Contingencies	9 Months Ended
	Sep. 30, 2021
Commitments and Contingencies
Commitments and Contingencies	9. Commitments and Contingencies ​ Claims and Lawsuits ​ We are subject to certain legal and regulatory claims, including lawsuits arising in the normal course of business. We maintain various insurance coverages to minimize financial risk associated with these claims. We have estimated and provided accruals for probable losses and related legal fees associated with certain litigation in the accompanying consolidated financial statements. While we cannot predict the outcome of these proceedings, in management’s opinion and based on reports of counsel, any liability arising from these matters individually and in the aggregate will not have a material effect on our operating results, cash flows or financial condition, after giving effect to provisions already recorded. ​ We are in a dispute with a customer regarding the outcome of a completed project and also regarding the obligation to perform subcontract work under two executed letters of intent for subsequent projects that we believe are not enforceable. The customer is claiming approximately $15 million in damages related to performance of the original project as well as excess costs to perform the work that was subject to the letters of intent. We are claiming approximately $9 million composed of unpaid amounts under the completed contract as well as costs and inefficiencies that we suffered. We have a lien on the project, and this matter is currently scheduled for arbitration in the second quarter of 2022 with a likely decision in the following months. As of September 30, 2021, we recorded an accrual for this matter based on our analysis of likely outcomes related to this dispute; however, it is possible that the ultimate outcome and associated costs will deviate from our estimates and that, in the event of an unexpectedly adverse outcome, we may experience additional costs and expenses in future periods. Surety ​ Many customers, particularly in connection with new construction, require us to post performance and payment bonds issued by a financial institution known as a surety. If we fail to perform under the terms of a contract or to pay subcontractors and vendors who provided goods or services under a contract, the customer may demand that the surety make payments or provide services under the bond. We must reimburse the surety for any expenses or outlays it incurs. To date, we are not aware of any losses to our sureties in connection with bonds the sureties have posted on our behalf and do not expect such losses to be incurred in the foreseeable future. ​ Current market conditions for surety markets and bonding capacity are adequate, with acceptable terms and conditions. Historically, approximately 15% to 25% of our business has required bonds. While we currently have strong surety relationships to support our bonding needs, future market conditions or changes in the sureties’ assessment of our operating and financial risk could cause the sureties to decline to issue bonds for our work. If that were to occur, the alternatives include doing more business that does not require bonds, posting other forms of collateral for project performance, such as letters of credit or cash, and seeking bonding capacity from other sureties. We would likely also encounter concerns from customers, suppliers and other market participants as to our creditworthiness. While we believe our general operating and financial characteristics would enable us to ultimately respond effectively to an interruption in the availability of bonding capacity, such an interruption would likely cause our revenue and profits to decline in the near term. ​ Self-Insurance ​ We are substantially self-insured for workers’ compensation, employer’s liability, auto liability, general liability and employee group health claims, in view of the relatively high per-incident deductibles we absorb under our insurance arrangements for these risks. Losses are estimated and accrued based upon known facts, historical trends and industry averages. Estimated losses in excess of our deductible, which have not already been paid, are included in our accrual with a corresponding receivable from our insurance carrier. Loss estimates associated with the larger and longer-developing risks, such as workers’ compensation, auto liability and general liability, are reviewed by a third-party actuary quarterly. ​

Stockholders' Equity

Stockholders' Equity	9 Months Ended
	Sep. 30, 2021
Stockholders' Equity
Stockholders' Equity.	10. Stockholders’ Equity ​ Earnings Per Share ​ Basic earnings per share (“EPS”) is computed by dividing net income by the weighted average number of shares of common stock outstanding during the year. Diluted EPS is computed considering the dilutive effect of stock options, restricted stock, restricted stock units and performance stock units. The vesting of unvested, contingently issuable performance stock units is based on the achievement of certain earnings per share targets and total shareholder return. These shares are considered contingently issuable shares for purposes of calculating diluted earnings per share. These shares are not included in the diluted earnings per share denominator until the performance criteria are met, if it is assumed that the end of the reporting period was the end of the contingency period. ​ Unvested restricted stock, restricted stock units and performance stock units are included in diluted earnings per share, weighted outstanding until the shares and units vest. Upon vesting, the vested restricted stock, restricted stock units and performance stock units are included in basic earnings per share weighted outstanding from the vesting date. ​ There were less than 0.1 million anti-dilutive stock options excluded from the calculation of diluted EPS for the three and nine months ended September 30, 2021 and 2020, respectively. ​ The following table reconciles the number of shares outstanding with the number of shares used in computing basic and diluted earnings per share for each of the periods presented (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Nine Months Ended ​ ​ ​ September 30, ​ September 30, ​ ​ 2021 2020 2021 2020 Common shares outstanding, end of period ​ 36,042 36,400 ​ 36,042 36,400 ​ Effect of using weighted average common shares outstanding ​ 254 157 ​ 286 204 ​ Shares used in computing earnings per share—basic ​ 36,296 36,557 ​ 36,328 36,604 ​ Effect of shares issuable under stock option plans based on the treasury stock method ​ 73 125 ​ 100 124 ​ Effect of restricted and contingently issuable shares ​ 65 68 ​ 72 69 ​ Shares used in computing earnings per share—diluted ​ 36,434 36,750 ​ 36,500 36,797 ​ ​ Share Repurchase Program ​ On March 29, 2007, our Board of Directors (the “Board”) approved a stock repurchase program to acquire up to 1.0 million shares of our outstanding common stock. Subsequently, the Board has from time to time increased the number of shares that may be acquired under the program and approved extensions of the program. On December 8, 2020, the Board approved an extension to the program by increasing the shares authorized for repurchase by 0.7 million shares. Since the inception of the repurchase program, the Board has approved 10.3 million shares to be repurchased. As of September 30, 2021, we have repurchased a cumulative total of 9.7 million shares at an average price of $21.57 per share under the repurchase program. ​ The share repurchases will be made from time to time at our discretion in the open market or privately negotiated transactions as permitted by securities laws and other legal requirements, and subject to market conditions and other factors. The Board may modify, suspend, extend or terminate the program at any time. During the nine months ended September 30, 2021, we repurchased 0.3 million shares for approximately $25.5 million at an average price of $73.69 per share.

Segment Information

Segment Information	9 Months Ended
	Sep. 30, 2021
Segment Information
Segment Information	11. Segment Information Our activities are within the mechanical services industry and the electrical services industry, which represent our two reportable segments. We aggregate our operating segments into two reportable segments, as the operating segments meet all of the aggregation criteria. On January 1, 2021, we completed a change in the structure of our internal organization that resulted in a portion of our electrical segment’s business being moved to the mechanical segment. We reallocated goodwill between our reorganized reporting units and have restated comparative prior period information for the reorganized reportable segments in accordance with the guidance on segment reporting. Accordingly, we have restated comparative prior period information for the reorganized reportable segments in the tables below. The following table presents information about our reportable segments (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, 2021 ​ Mechanical Services Electrical Services Corporate Consolidated Revenue ​ $ 690,680 ​ $ 143,216 ​ $ — ​ $ 833,896 Gross Profit ​ $ 138,618 ​ $ 20,594 ​ $ — ​ $ 159,212 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, 2020 ​ Mechanical Services Electrical Services Corporate Consolidated Revenue ​ $ 625,660 ​ $ 88,439 ​ $ — ​ $ 714,099 Gross Profit ​ $ 139,845 ​ $ 7,351 ​ $ — ​ $ 147,196 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Nine Months Ended September 30, 2021 ​ Mechanical Services Electrical Services Corporate Consolidated Revenue ​ $ 1,868,096 ​ $ 349,456 ​ $ — ​ $ 2,217,552 Gross Profit ​ $ 359,151 ​ $ 49,985 ​ $ — ​ $ 409,136 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Nine Months Ended September 30, 2020 ​ Mechanical Services Electrical Services Corporate Consolidated Revenue ​ $ 1,823,117 ​ $ 334,581 ​ $ — ​ $ 2,157,698 Gross Profit ​ $ 386,537 ​ $ 23,447 ​ $ — ​ $ 409,984 ​

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	9 Months Ended
	Sep. 30, 2021
Summary of Significant Accounting Policies
Basis of Presentation	Basis of Presentation ​ These interim statements should be read in conjunction with the historical Consolidated Financial Statements and related notes of Comfort Systems included in the Annual Report on Form 10-K as filed with the Securities and Exchange Commission (“SEC”) for the year ended December 31, 2020 (the “Form 10-K”). ​ The accompanying unaudited consolidated financial statements were prepared using generally accepted accounting principles for interim financial information and the instructions to Form 10-Q and applicable rules of Regulation S-X of the SEC. Accordingly, these financial statements do not include all the footnotes required by generally accepted accounting principles for complete financial statements and should be read in conjunction with the Form 10-K. We believe all adjustments necessary for a fair presentation of these interim statements have been included and are of a normal and recurring nature. The results of operations for interim periods are not necessarily indicative of the results for the full fiscal year.
Use of Estimates	Use of Estimates ​ The preparation of financial statements in conformity with generally accepted accounting principles requires the use of estimates and assumptions by management in determining the reported amounts of assets and liabilities, revenue and expenses and disclosures regarding contingent assets and liabilities. Actual results could differ from those estimates. The most significant estimates used in our financial statements affect revenue and cost recognition for construction contracts, self-insurance accruals, deferred tax assets, fair value accounting for acquisitions and the quantification of fair value for reporting units in connection with our goodwill impairment testing. ​
Recent Accounting Pronouncements	Recent Accounting Pronouncements ​ In December 2019, the FASB issued ASU No. 2019-12, “Income Taxes (Topic 740): Simplifying the Accounting for Income Taxes.” This standard simplifies the accounting for income taxes by eliminating certain exceptions to the guidance in Topic 740 related to the approach for intraperiod tax allocation, the methodology for calculating income taxes in an interim period and the recognition of deferred tax liabilities for outside basis differences. The standard also simplifies aspects of the accounting for franchise taxes and enacted changes in tax laws or rates and clarifies the accounting for transactions that result in a step-up in the tax basis of goodwill. ASU No. 2019-12 is effective for fiscal years beginning after December 15, 2020 and interim periods within that year. We adopted ASU No. 2019-12 on January 1, 2021, and the impact was not material to our overall financial statements. ​
Revenue Recognition	Revenue Recognition ​ We recognize revenue over time for all of our services as we perform them because (i) control continuously transfers to that customer as work progresses, and (ii) we have the right to bill the customer as costs are incurred. The customer typically controls the work in process, as evidenced either by contractual termination clauses or by our rights to payment for work performed to date, plus a reasonable profit, for delivery of products or services that do not have an alternative use to the Company. ​ For the reasons listed above, revenue is recognized based on the extent of progress towards completion of the performance obligation. The selection of the method to measure progress towards completion requires judgment and is based on the nature of the products or services to be provided. We generally use the cost to cost measure of progress for our contracts, as it best depicts the transfer of assets to the customer that occurs as we incur costs on our contracts. Under the cost to cost measure of progress, the extent of progress towards completion is measured based on the ratio of costs incurred to date to the total estimated costs at completion of the performance obligation. Revenue, including estimated fees or profits, is recorded proportionally as costs are incurred. Costs to fulfill include labor, materials, subcontractors’ costs, other direct costs and an allocation of indirect costs. ​ For a small portion of our business in which our services are delivered in the form of service maintenance agreements for existing systems to be repaired and maintained, as opposed to constructed, our performance obligation is to maintain the customer’s mechanical system for a specific period of time. Similar to construction jobs, we recognize revenue over time; however, for service maintenance agreements in which the full cost to provide services may not be known, we generally use an input method to recognize revenue, which is based on the amount of time we have provided our services out of the total time we have been contracted to perform those services. Our revenue recognition policy is further discussed in Note 3 “Revenue from Contracts with Customers.”
Accounts Receivable and Allowance for Credit Losses	Accounts Receivable and Allowance for Credit Losses ​ We are required to estimate and record the expected credit losses over the contractual life of our financial assets measured at amortized cost, including billed and unbilled accounts receivable, other receivables and costs and estimated earnings in excess of billings. Accounts receivable include amounts from work completed in which we have billed or have an unconditional right to bill our customers. Our trade receivables are contractually due in less than a year. ​ We estimate our credit losses using a loss-rate method for each of our identified portfolio segments. Our portfolio segments are construction, service and other. While our construction and service financial assets are often with the same subset of customers and industries, our construction financial assets will generally have a lower loss-rate than service financial assets due to lien rights, which we are more likely to have on construction jobs. These lien rights result in lower credit loss expenses on average compared to receivables that do not have lien rights. Financial assets classified as Other include receivables that are not related to our core revenue producing activities, such as receivables related to our acquisition activity from former owners, our vendor rebate program or receivables for estimated losses in excess of our insurance deductible, which are accrued with a corresponding accrued insurance liability. ​ Loss rates for our portfolios are based on numerous factors, including our history of credit loss expense by portfolio, the financial strength of our customers and counterparties in each portfolio, the aging of our receivables, our expectation of likelihood of payment, macroeconomic trends in the U.S. and the current and forecasted non-residential construction market trends in the U.S. ​ In addition to the loss-rate calculations discussed above, we also record allowance for credit losses for specific receivables that are deemed to have a higher risk profile than the rest of the respective pool of receivables (e.g., when we hold concerns about a specific customer going bankrupt and no longer being able to pay the receivables due to us).
Income Taxes	Income Taxes ​ We conduct business throughout the United States in virtually all fifty states. Our effective tax rate changes based upon our relative profitability, or lack thereof, in states with varying tax rates and rules. In addition, discrete items, such as tax law changes, judgments and legal structures, can impact our effective tax rate. These items can also include the tax treatment for impairment of goodwill and other intangible assets, changes in fair value of acquisition-related assets and liabilities, tax reserves for uncertain tax positions and accounting for losses associated with underperforming operations. ​ During the third quarter of 2020, the Internal Revenue Service (the “IRS”) completed its examination of our amended federal returns for 2014 and 2015 and issued a Revenue Agent Report allowing the $8.9 million of refund claims in full. Subsequently, the Joint Committee on Taxation reviewed and approved the refund claims. As a result, our provision for income taxes was reduced by $8.3 million due to a decrease in unrecognized tax benefits. ​ In early October 2020, we filed amended federal returns for 2016, 2017 and 2018 to claim the credit for increasing research activities (the “R&D tax credit”) and the energy efficient commercial buildings deduction (the “179D deduction”) allocated to us and recorded tax benefits of $6.1 million, $8.5 million and $11.9 million, respectively. The $26.5 million of tax benefits was offset by additions to unrecognized tax benefits of $26.4 million due to the uncertainty of the outcome of our current IRS examination. The R&D tax credit and 179D deduction for 2016, 2017 and 2018, therefore, had no material impact on our effective tax rates. ​
Financial Instruments	Financial Instruments ​ Our financial instruments consist of cash and cash equivalents, accounts receivable, other receivables, accounts payable, interest rate swaps, life insurance policies, notes to former owners, a revolving credit facility and a term loan. We believe that the carrying values of these instruments in the accompanying Balance Sheets approximate their fair values.
Leases	We lease certain facilities, vehicles and equipment primarily under noncancelable operating leases. The most significant portion of these noncancelable operating leases are for the facilities occupied by our corporate office and our operating locations. As a result of our acquisition of Amteck in August 2021, we acquired some finance leases on vehicles that are not material to our consolidated financial position. As of September 30, 2021, the lease liabilities associated with these finance leases was $2.4 million. Leases with an initial term of 12 months or less are not recorded in the Balance Sheet. We do not separate lease components from their associated non-lease components pursuant to lease accounting guidance. We have certain leases with variable payments based on an index as well as some short-term leases on equipment and facilities. Variable lease expense and short-term lease expense were not material to our financial statements and aggregated to $5.7 million and $6.1 million in the first nine months of 2021 and 2020, respectively. Lease right-of-use assets and liabilities are recognized at commencement date based on the present value of lease payments over the lease term. As most of our leases do not provide an implicit rate, we generally use our incremental borrowing rate based on the information available at commencement date in determining the present value of lease payments. The weighted average discount rate for our operating leases as of September 30, 2021 and December 31, 2020 was 4.1% and 4.2%, respectively. We recognize operating lease expense, including escalating lease payments and lease incentives, on a straight-line basis over the lease term. Operating lease expense for the three months ended September 30, 2021 and 2020 was $7.7 million and $8.0 million, respectively. Operating lease expense for the nine months ended September 30, 2021 and 2020 was $22.1 million and $21.4 million, respectively. ​ The lease terms generally range from three ​ A majority of the Company’s real property leases are with individuals or entities with whom we have no other business relationship. However, in certain instances the Company enters into real property leases with current or former employees. Rent paid to related parties for the three months ended September 30, 2021 and 2020 was approximately $1.1 million and $2.0 million, respectively. Rent paid to related parties for the nine months ended September 30, 2021 and 2020 was approximately $3.6 million and $4.5 million, respectively. ​ If we decide to cancel or terminate a lease before the end of its term, we would typically owe the lessor the remaining lease payments under the term of the lease. Our lease agreements do not contain any material residual value guarantees or material restrictive covenants. On rare occasions, we rent or sublease certain real estate assets that we no longer use to third parties.

Revenue from Contracts with C_2

Revenue from Contracts with Customers (Tables)	9 Months Ended
	Sep. 30, 2021
Revenue from Contract with Customers
Schedule of disaggregation of revenue	See details in the following tables (dollars in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ ​ Nine Months Ended September 30, Revenue by Service Provided 2021 ​ 2020 ​ 2021 ​ 2020 ​ Mechanical Services ​ $ 690,680 82.8 % ​ $ 625,660 87.6 % ​ $ 1,868,096 84.2 % ​ $ 1,823,117 84.5 % Electrical Services ​ ​ 143,216 ​ 17.2 % ​ ​ 88,439 ​ 12.4 % ​ ​ 349,456 ​ 15.8 % ​ ​ 334,581 ​ 15.5 % Total ​ $ 833,896 ​ 100.0 % ​ $ 714,099 ​ 100.0 % ​ $ 2,217,552 ​ 100.0 % ​ $ 2,157,698 ​ 100.0 % ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ ​ Nine Months Ended September 30, Revenue by Type of Customer ​ 2021 ​ ​ 2020 ​ 2021 ​ ​ 2020 Industrial ​ $ 379,605 ​ 45.5 % ​ $ 268,651 ​ 37.6 % ​ $ 960,263 ​ 43.3 % ​ $ 844,719 ​ 39.1 % Education ​ ​ 115,536 ​ 13.9 % ​ ​ 135,992 ​ 19.0 % ​ ​ 300,374 ​ 13.5 % ​ ​ 375,580 ​ 17.4 % Office Buildings ​ ​ 77,806 ​ 9.3 % ​ ​ 81,183 ​ 11.4 % ​ ​ 229,816 ​ 10.4 % ​ ​ 232,349 ​ 10.8 % Healthcare ​ ​ 101,446 ​ 12.2 % ​ ​ 86,061 ​ 12.1 % ​ ​ 292,541 ​ 13.2 % ​ ​ 281,370 ​ 13.0 % Government ​ ​ 40,268 ​ 4.8 % ​ ​ 40,631 ​ 5.7 % ​ ​ 125,939 ​ 5.7 % ​ ​ 119,444 ​ 5.5 % Retail, Restaurants and Entertainment ​ ​ 62,365 ​ 7.5 % ​ ​ 64,212 ​ 9.0 % ​ ​ 155,874 ​ 7.0 % ​ ​ 190,043 ​ 8.8 % Multi-Family and Residential ​ ​ 29,741 ​ 3.6 % ​ ​ 21,776 ​ 3.0 % ​ ​ 82,742 ​ 3.7 % ​ ​ 61,062 ​ 2.8 % Other ​ ​ 27,129 ​ 3.2 % ​ ​ 15,593 ​ 2.2 % ​ ​ 70,003 ​ 3.2 % ​ ​ 53,131 ​ 2.6 % Total ​ $ 833,896 ​ 100.0 % ​ $ 714,099 ​ 100.0 % ​ $ 2,217,552 ​ 100.0 % ​ $ 2,157,698 ​ 100.0 % ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ ​ Nine Months Ended September 30, Revenue by Activity Type ​ 2021 ​ ​ 2020 ​ 2021 ​ ​ 2020 New Construction ​ $ 397,446 ​ 47.6 % ​ $ 310,000 ​ 43.4 % ​ $ 1,029,397 ​ 46.4 % ​ $ 1,034,833 ​ 48.0 % Existing Building Construction ​ ​ 250,893 ​ 30.1 % ​ ​ 235,798 ​ 33.0 % ​ ​ 682,811 ​ 30.8 % ​ ​ 668,067 ​ 30.9 % Service Projects ​ ​ 75,525 ​ 9.1 % ​ ​ 69,162 ​ 9.7 % ​ ​ 201,848 ​ 9.1 % ​ ​ 179,188 ​ 8.3 % Service Calls, Maintenance and Monitoring ​ ​ 110,032 ​ 13.2 % ​ ​ 99,139 ​ 13.9 % ​ ​ 303,496 ​ 13.7 % ​ ​ 275,610 ​ 12.8 % Total ​ $ 833,896 ​ 100.0 % ​ $ 714,099 ​ 100.0 % ​ $ 2,217,552 ​ 100.0 % ​ $ 2,157,698 ​ 100.0 % ​
Schedule of contract assets and liabilities	The following table presents the changes in contract assets and contract liabilities (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Nine Months Ended September 30, ​ ​ Year Ended December 31, ​ 2021 ​ 2020 ​ Contract Contract ​ Contract Contract ​ Assets ​ Liabilities ​ Assets ​ Liabilities Balance at beginning of period $ 18,622 ​ $ 226,237 ​ $ 2,736 ​ $ 166,918 Change due to acquisitions / disposals ​ 2,378 ​ ​ 18,084 ​ ​ 9,509 ​ ​ 39,885 Change related to credit allowance ​ 45 ​ ​ — ​ ​ (79) ​ ​ — Other changes in the period ​ (6,073) ​ ​ 257 ​ ​ 6,456 ​ ​ 19,434 Balance at end of period $ 14,972 ​ $ 244,578 ​ $ 18,622 $ 226,237 ​

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	9 Months Ended
	Sep. 30, 2021
Fair Value Measurements
Summary of fair values and levels within the fair value hierarchy in which the fair value measurements fall for assets and liabilities measured on a recurring basis	The following table summarizes the fair values, and levels within the fair value hierarchy in which the fair value measurements fall, for assets and liabilities measured on a recurring basis as of September 30, 2021 and December 31, 2020 (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at September 30, 2021 ​ Level 1 Level 2 Level 3 Total Cash and cash equivalents ​ $ 67,721 ​ $ — ​ $ — ​ $ 67,721 Life insurance—cash surrender value ​ $ — ​ $ 6,683 ​ $ — ​ $ 6,683 Contingent earn-out obligations ​ $ — ​ $ — ​ $ 33,780 ​ $ 33,780 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair Value Measurements at December 31, 2020 ​ Level 1 Level 2 Level 3 Total Cash and cash equivalents ​ $ 54,896 ​ $ — ​ $ — ​ $ 54,896 Life insurance—cash surrender value ​ $ — ​ $ 5,420 ​ $ — ​ $ 5,420 Contingent earn-out obligations ​ $ — ​ $ — ​ $ 25,979 ​ $ 25,979 Interest rate swap liability ​ $ — ​ $ 42 ​ $ — ​ $ 42 ​
Schedule of reconciliation of the fair value of contingent earn-out obligations that use significant unobservable inputs (Level 3)	The table below presents a reconciliation of the fair value of our contingent earn-out obligations that use significant unobservable inputs (Level 3) (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Nine Months Ended ​ Year Ended ​ ​ September 30, 2021 ​ December 31, 2020 ​ Balance at beginning of period $ 25,979 ​ $ 28,497 Issuances ​ 16,155 ​ 16,715 ​ Settlements ​ ​ (3,831) ​ ​ (10,114) ​ Adjustments to fair value ​ (4,523) ​ (9,119) ​ Balance at end of period ​ $ 33,780 ​ $ 25,979 ​

Acquisitions (Tables)

Acquisitions (Tables)	9 Months Ended
	Sep. 30, 2021
Acquisitions
Schedule of acquisition date fair value of consideration transferred and the acquisition date fair value of the identifiable assets acquired and liabilities assumed, including an amount for goodwill	The following summarizes the acquisition date fair value of consideration transferred and the acquisition date fair value of the identifiable assets acquired and liabilities assumed, including an amount for goodwill (in thousands): ​ ​ ​ ​ Consideration transferred: ​ ​ Cash paid at closing $ 107,355 Working capital adjustment ​ 6,892 Notes issued to former owners ​ 10,000 Estimated fair value of contingent earn-out payments ​ 12,900 ​ $ 137,147 Recognized amounts of identifiable assets acquired and liabilities assumed: ​ ​ Cash and cash equivalents $ 13,554 Billed and unbilled accounts receivable ​ 43,248 Other current assets ​ 3,422 Property and equipment ​ 3,891 Goodwill ​ 54,554 Identifiable intangible assets ​ 57,700 Operating lease right-of-use asset ​ 8,997 Accounts payable ​ (14,489) Billings in excess of costs and estimated earnings ​ (15,199) Current maturities of long-term debt ​ (1,860) Current operating lease liabilities ​ (1,115) Accrued expenses and other current liabilities ​ (6,889) Long-term debt ​ (785) Long-term operating lease liabilities ​ (7,882) ​ $ 137,147
Schedule of acquired intangible assets	The acquired intangible assets include the following (dollars in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Valuation Method ​ Estimated Useful Life ​ ​ Estimated Fair Value Backlog ​ Excess earnings ​ 1.5 years ​ $ 7,800 Trade Name ​ Relief-from-royalty ​ 23 years ​ ​ 12,600 Customer Relationships ​ Excess earnings ​ 10 years ​ ​ 37,300 Total ​ ​ ​ ​ ​ $ 57,700

Goodwill and Identifiable Int_2

Goodwill and Identifiable Intangible Assets, Net (Tables)	9 Months Ended
	Sep. 30, 2021
Goodwill and Identifiable Intangible Assets, Net
Schedule of changes in the carrying amount of goodwill	The changes in the carrying amount of goodwill are as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Mechanical Services ​ Electrical Services ​ ​ ​ Segment Segment ​ Total Balance at December 31, 2019 ​ $ 234,660 ​ $ 97,787 ​ $ 332,447 Acquisitions and purchase price adjustments (See Note 5) ​ 72,788 ​ ​ 59,157 ​ ​ 131,945 Balance at December 31, 2020 ​ ​ 307,448 ​ ​ 156,944 ​ ​ 464,392 Acquisitions and purchase price adjustments (See Note 5) ​ ​ 8,089 ​ ​ 54,483 ​ ​ 62,572 Impact of segment reorganization ​ ​ 1,101 ​ ​ (1,101) ​ ​ — Balance at September 30, 2021 ​ $ 316,638 ​ $ 210,326 ​ $ 526,964
Schedule of future amortization expense of identifiable intangible assets	At September 30, 2021, future amortization expense of identifiable intangible assets is as follows (in thousands): ​ ​ ​ ​ ​ ​ Year ended December 31— ​ ​ 2021 (remainder of the year) $ 11,021 2022 ​ ​ 36,968 ​ 2023 ​ 28,495 ​ 2024 ​ 27,145 ​ 2025 ​ 24,958 ​ Thereafter ​ 139,512 ​ Total ​ $ 268,099 ​

Debt Obligations (Tables)

Debt Obligations (Tables)	9 Months Ended
	Sep. 30, 2021
Debt Obligations
Schedule of components of debt obligations	Debt obligations consist of the following (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ September 30, ​ December 31, ​ ​ 2021 2020 Revolving credit facility ​ $ 115,000 ​ $ 70,000 ​ Term loan ​ ​ 120,000 ​ ​ 135,000 ​ Notes to former owners ​ ​ 35,500 ​ 31,000 ​ Finance lease liabilities (See Note 8) ​ ​ 2,438 ​ ​ — ​ Total principal amount ​ ​ 272,938 ​ 236,000 ​ Less—unamortized debt issuance costs ​ ​ (208) ​ ​ (267) ​ Total debt, net of unamortized debt issuance costs ​ ​ 272,730 ​ ​ 235,733 ​ Less—current portion ​ ​ (5,958) ​ — ​ Total long-term portion of debt, net ​ $ 266,772 ​ $ 235,733 ​ ​
Summary of additional margins	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Consolidated Total Indebtedness to ​ ​ Credit Facility Adjusted EBITDA ​ Less than 1.00 1.00 to 1.75 1.75 to 2.50 2.50 or greater Additional Per Annum Interest Margin Added Under: ​ ​ ​ ​ ​ ​ ​ ​ ​ Base Rate Loan Option ​ 0.25 % 0.50 % 0.75 % 1.00 % Eurodollar Rate Loan Option ​ 1.25 % 1.50 % 1.75 % 2.00 %

Leases (Tables)

Leases (Tables)	9 Months Ended
	Sep. 30, 2021
Leases
Schedule of lease assets and liabilities	Finance lease right-of-use assets are included in “Property and equipment, net,” and current and long-term finance lease liabilities are included within “Current maturities of long-term debt” and “Long-term debt, net,” respectively, in the consolidated Balance Sheet. The following table summarizes the operating lease assets and liabilities included in the consolidated Balance Sheet as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ September 30, 2021 ​ December 31, 2020 Operating lease right-of-use assets $ 104,336 ​ $ 94,727 Operating lease liabilities: ​ ​ ​ ​ ​ Other current liabilities $ 17,499 ​ $ 16,586 Long-term operating lease liabilities ​ 88,923 ​ ​ 80,576 Total operating lease liabilities $ 106,422 ​ $ 97,162
Schedule of maturities of lease liabilities	The maturities of operating lease liabilities are as follows (in thousands): ​ ​ ​ ​ ​ Year ending December 31— ​ ​ ​ 2021 (excluding the nine months ended September 30, 2021) ​ $ 5,668 2022 ​ ​ 20,771 2023 ​ ​ 18,101 2024 ​ ​ 16,066 2025 ​ ​ 15,180 Thereafter ​ ​ 49,092 Total Lease Payments ​ ​ 124,878 Less—Present Value Discount ​ ​ (18,456) Present Value of Operating Lease Liabilities ​ $ 106,422
Schedule of supplemental information related to leases	Supplemental information related to operating leases was as follows (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, ​ Nine Months Ended September 30, ​ 2021 ​ 2020 2021 ​ 2020 Cash paid for amounts included in the measurement of operating lease liabilities $ 5,597 ​ $ 5,362 ​ $ 16,297 ​ $ 15,140 Operating lease right-of-use assets obtained in exchange for lease liabilities $ 13,154 ​ $ 1,880 ​ $ 22,513 ​ $ 24,080

Stockholders' Equity (Tables)

Stockholders' Equity (Tables)	9 Months Ended
	Sep. 30, 2021
Stockholders' Equity
Reconciliation of number of shares outstanding with the number of shares used in computing basic and diluted earnings per share	The following table reconciles the number of shares outstanding with the number of shares used in computing basic and diluted earnings per share for each of the periods presented (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended ​ Nine Months Ended ​ ​ ​ September 30, ​ September 30, ​ ​ 2021 2020 2021 2020 Common shares outstanding, end of period ​ 36,042 36,400 ​ 36,042 36,400 ​ Effect of using weighted average common shares outstanding ​ 254 157 ​ 286 204 ​ Shares used in computing earnings per share—basic ​ 36,296 36,557 ​ 36,328 36,604 ​ Effect of shares issuable under stock option plans based on the treasury stock method ​ 73 125 ​ 100 124 ​ Effect of restricted and contingently issuable shares ​ 65 68 ​ 72 69 ​ Shares used in computing earnings per share—diluted ​ 36,434 36,750 ​ 36,500 36,797 ​ ​

Segment Information (Tables)

Segment Information (Tables)	9 Months Ended
	Sep. 30, 2021
Segment Information
Summary of information about reportable segments	The following table presents information about our reportable segments (in thousands): ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, 2021 ​ Mechanical Services Electrical Services Corporate Consolidated Revenue ​ $ 690,680 ​ $ 143,216 ​ $ — ​ $ 833,896 Gross Profit ​ $ 138,618 ​ $ 20,594 ​ $ — ​ $ 159,212 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Three Months Ended September 30, 2020 ​ Mechanical Services Electrical Services Corporate Consolidated Revenue ​ $ 625,660 ​ $ 88,439 ​ $ — ​ $ 714,099 Gross Profit ​ $ 139,845 ​ $ 7,351 ​ $ — ​ $ 147,196 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Nine Months Ended September 30, 2021 ​ Mechanical Services Electrical Services Corporate Consolidated Revenue ​ $ 1,868,096 ​ $ 349,456 ​ $ — ​ $ 2,217,552 Gross Profit ​ $ 359,151 ​ $ 49,985 ​ $ — ​ $ 409,136 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Nine Months Ended September 30, 2020 ​ Mechanical Services Electrical Services Corporate Consolidated Revenue ​ $ 1,823,117 ​ $ 334,581 ​ $ — ​ $ 2,157,698 Gross Profit ​ $ 386,537 ​ $ 23,447 ​ $ — ​ $ 409,984

Summary of Significant Accoun_3

Summary of Significant Accounting Policies - Income Taxes (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Oct. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Income Tax Examination [Line Items]
Reduced provision for income taxes			$ 8,300
Provision for income taxes		$ 14,999	8,279	$ 33,553	$ 30,100
Additions based on tax positions related to prior years	$ 26,400
Tax Year 2016
Income Tax Examination [Line Items]
Provision for income taxes	6,100
Tax Year 2017
Income Tax Examination [Line Items]
Provision for income taxes	8,500
Tax Year 2018
Income Tax Examination [Line Items]
Provision for income taxes	11,900
Total Tax Years 2016-2018
Income Tax Examination [Line Items]
Provision for income taxes	$ 26,500
Internal Revenue Service (IRS)
Income Tax Examination [Line Items]
Refund claims allowed in Income tax examination			$ 8,900		$ 8,900

Revenue from Contracts with C_3

Revenue from Contracts with Customers - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Disaggregation of Revenue
Revenue	$ 833,896	$ 714,099	$ 2,217,552	$ 2,157,698
Percentage of revenue from contract with customer (as a percent)	100.00%	100.00%	100.00%	100.00%
Industrial
Disaggregation of Revenue
Revenue	$ 379,605	$ 268,651	$ 960,263	$ 844,719
Percentage of revenue from contract with customer (as a percent)	45.50%	37.60%	43.30%	39.10%
Education
Disaggregation of Revenue
Revenue	$ 115,536	$ 135,992	$ 300,374	$ 375,580
Percentage of revenue from contract with customer (as a percent)	13.90%	19.00%	13.50%	17.40%
Office Buildings
Disaggregation of Revenue
Revenue	$ 77,806	$ 81,183	$ 229,816	$ 232,349
Percentage of revenue from contract with customer (as a percent)	9.30%	11.40%	10.40%	10.80%
Healthcare
Disaggregation of Revenue
Revenue	$ 101,446	$ 86,061	$ 292,541	$ 281,370
Percentage of revenue from contract with customer (as a percent)	12.20%	12.10%	13.20%	13.00%
Government
Disaggregation of Revenue
Revenue	$ 40,268	$ 40,631	$ 125,939	$ 119,444
Percentage of revenue from contract with customer (as a percent)	4.80%	5.70%	5.70%	5.50%
Retail, Restaurants and Entertainment
Disaggregation of Revenue
Revenue	$ 62,365	$ 64,212	$ 155,874	$ 190,043
Percentage of revenue from contract with customer (as a percent)	7.50%	9.00%	7.00%	8.80%
Multi-Family and Residential
Disaggregation of Revenue
Revenue	$ 29,741	$ 21,776	$ 82,742	$ 61,062
Percentage of revenue from contract with customer (as a percent)	3.60%	3.00%	3.70%	2.80%
Other
Disaggregation of Revenue
Revenue	$ 27,129	$ 15,593	$ 70,003	$ 53,131
Percentage of revenue from contract with customer (as a percent)	3.20%	2.20%	3.20%	2.60%
New Construction
Disaggregation of Revenue
Revenue	$ 397,446	$ 310,000	$ 1,029,397	$ 1,034,833
Percentage of revenue from contract with customer (as a percent)	47.60%	43.40%	46.40%	48.00%
Existing Building Construction
Disaggregation of Revenue
Revenue	$ 250,893	$ 235,798	$ 682,811	$ 668,067
Percentage of revenue from contract with customer (as a percent)	30.10%	33.00%	30.80%	30.90%
Service Projects
Disaggregation of Revenue
Revenue	$ 75,525	$ 69,162	$ 201,848	$ 179,188
Percentage of revenue from contract with customer (as a percent)	9.10%	9.70%	9.10%	8.30%
Service Calls, Maintenance and Monitoring
Disaggregation of Revenue
Revenue	$ 110,032	$ 99,139	$ 303,496	$ 275,610
Percentage of revenue from contract with customer (as a percent)	13.20%	13.90%	13.70%	12.80%
Mechanical Services
Disaggregation of Revenue
Revenue	$ 690,680	$ 625,660	$ 1,868,096	$ 1,823,117
Percentage of revenue from contract with customer (as a percent)	82.80%	87.60%	84.20%	84.50%
Electrical Services
Disaggregation of Revenue
Revenue	$ 143,216	$ 88,439	$ 349,456	$ 334,581
Percentage of revenue from contract with customer (as a percent)	17.20%	12.40%	15.80%	15.50%

Revenue from Contracts with C_4

Revenue from Contracts with Customers - Contract Assets and Liabilities (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Contract Assets
Balance at beginning of period	$ 18,622	$ 2,736	$ 2,736
Change due to acquisitions / disposals	2,378		9,509
Change related to credit allowance	45		(79)
Other changes in the period	(6,073)		6,456
Balance at end of period	14,972		18,622
Contract Liabilities
Balance at beginning of period	226,237	166,918	166,918
Change due to acquisitions / disposals	18,084		39,885
Other changes in the period	257		19,434
Balance at end of period	244,578		$ 226,237
Revenue related to our contract liabilities	$ 204,600
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Contract Liabilities
Revenue related to our contract liabilities		$ 164,400

Revenue from Contracts with C_5

Revenue from Contracts with Customers - Remaining Performance Obligations (Details) $ in Millions	9 Months Ended
	Sep. 30, 2021USD ($)
Remaining Performance Obligations
The term of the renewable service maintenance agreements (in years)	1 year
Practical Expedient	true
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-10-01
Remaining Performance Obligations
Remaining performance obligations	$ 1,940
Expected timing of performance obligations	12 months
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-10-01 | Minimum
Remaining Performance Obligations
Expected percentage of remaining performance obligations	80.00%
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-10-01 | Maximum
Remaining Performance Obligations
Expected percentage of remaining performance obligations	85.00%

Fair Value Measurements (Detail

Fair Value Measurements (Details) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021USD ($)employee	Sep. 30, 2020USD ($)	Sep. 30, 2021USD ($)employee	Sep. 30, 2020USD ($)	Dec. 31, 2020USD ($)	Nov. 30, 2021USD ($)
Fair Value Measurements
Number of employees covered under life insurance policies | employee	89		89
Combined face value of life insurance policies	$ 63,900		$ 63,900
Cash surrender value	6,700		$ 6,700		$ 5,400
Minimum
Fair Value Measurements
Weighted average cost of capital			10.50%
Maximum
Fair Value Measurements
Weighted average cost of capital			17.00%
Contingent earn-out obligations
Reconciliation of the fair value of contingent earn-out obligations that use significant unobservable inputs (Level 3)
Balance at beginning of year			$ 25,979	$ 28,497	28,497
Issuances			16,155		16,715
Settlements			(3,831)		(10,114)
Adjustments to fair value			(4,523)		(9,119)
Balance at end of period	33,780		33,780		25,979
Recurring basis | Total
Fair Value Measurements
Cash and cash equivalents	67,721		67,721		54,896
Life insurance-cash surrender value	6,683		6,683		5,420
Contingent earn-out obligations	33,780		33,780		25,979
Recurring basis | Quoted Market Prices In Active Markets for Identical Assets (Level 1)
Fair Value Measurements
Cash and cash equivalents	67,721		67,721		54,896
Recurring basis | Significant Other Observable Inputs (Level 2)
Fair Value Measurements
Life insurance-cash surrender value	6,683		6,683		5,420
Recurring basis | Significant Unobservable Inputs (Level 3)
Fair Value Measurements
Contingent earn-out obligations	33,780		33,780		25,979
Interest Rate Swap
Fair Value Measurements
Net loss on derivative	100	$ 200	400	$ 200
Interest Rate Swap | Not Designated as Hedging Instrument, Economic Hedge [Member]
Fair Value Measurements
Notional amount	$ 130,000		$ 130,000			$ 80,000
Interest Rate Swap | Recurring basis | Total | Not Designated as Hedging Instrument, Economic Hedge [Member]
Fair Value Measurements
Interest rate swap liability					42
Interest Rate Swap | Recurring basis | Significant Other Observable Inputs (Level 2) | Not Designated as Hedging Instrument, Economic Hedge [Member]
Fair Value Measurements
Interest rate swap liability					$ 42

Acquisitions (Details)

Acquisitions (Details) - USD ($) $ in Thousands	Aug. 01, 2021	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Recognized amounts of identifiable assets acquired and liabilities assumed:
Goodwill		$ 526,964	$ 464,392	$ 332,447
Amteck
Consideration transferred:
Cash paid at closing	$ 107,355
Working capital adjustment	6,892
Notes issued to former owners	10,000
Estimated fair value of contingent earn-out payments	12,900
Total consideration transferred	137,147
Recognized amounts of identifiable assets acquired and liabilities assumed:
Cash and cash equivalents	13,554
Billed and unbilled accounts receivable	43,248
Other current assets	3,422
Property and equipment	3,891
Goodwill	54,554
Identifiable intangible assets	57,700
Operating lease right-of-use asset	8,997
Accounts payable	(14,489)
Billings in excess of costs and estimated earnings	(15,199)
Current maturities of long-term debt	(1,860)
Current operating lease liabilities	(1,115)
Accrued expenses and other current liabilities	(6,889)
Long-term debt	(785)
Long-term operating lease liabilities	(7,882)
Total assets acquired and liabilities assumed	$ 137,147

Acquisitions - Acquired Intangi

Acquisitions - Acquired Intangible Assets (Details) - Amteck $ in Thousands	Aug. 01, 2021USD ($)
Acquired Finite-Lived Intangible Assets [Line Items]
Estimated Fair Value	$ 57,700
Backlog
Acquired Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	1 year 6 months
Estimated Fair Value	$ 7,800
Trade Name
Acquired Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	23 years
Estimated Fair Value	$ 12,600
Customer Relationships
Acquired Finite-Lived Intangible Assets [Line Items]
Estimated Useful Life	10 years
Estimated Fair Value	$ 37,300

Acquisitions - Contingent Earn-

Acquisitions - Contingent Earn-out Obligation (Details) - USD ($) $ in Thousands	Aug. 01, 2021	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Business Acquisition [Line Items]
Revenues		$ 833,896	$ 714,099	$ 2,217,552	$ 2,157,698
Amteck
Business Acquisition [Line Items]
Number of milestones	four
Contingent earn-out period	53 months
Amteck | Minimum
Business Acquisition [Line Items]
Contingent earn-out estimated milestone payment, minimum	$ 100
Cash flow discount rate	14.80%
Amteck | Minimum | Measurement Input, Inception Discount Rate [Member]
Business Acquisition [Line Items]
Cash flow discount rate	14.00%
Amteck | Maximum
Business Acquisition [Line Items]
Contingent earn-out estimated milestone payment, maximum	$ 11,400
Cash flow discount rate	15.20%
Amteck | Maximum | Measurement Input, Inception Discount Rate [Member]
Business Acquisition [Line Items]
Cash flow discount rate	18.50%

Acquisitions - Other Acquisitio

Acquisitions - Other Acquisitions (Details) - USD ($) $ in Millions	Apr. 01, 2020	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
TAS Energy
Acquisitions
Total purchase price	$ 169.5
Purchase price allocated to goodwill and identifiable intangible assets	126.2
Purchase price paid in cash	105.9
Working capital adjustment	40.5
Promissory note payable	14
Contingent earn-out obligation	$ 9.1
T E C Industrial Construction and Maintenance
Acquisitions
Total purchase price			$ 88.9
Purchase price paid in cash			73
Working capital adjustment			1.3
Promissory note payable			7
Estimated fair value of contingent earn-out payments			$ 7.6
Electrical Contractor North Carolina
Acquisitions
Total purchase price				$ 41.6
Mechanical Contractor Utah
Acquisitions
Total purchase price		$ 17.9

Goodwill and Identifiable Int_3

Goodwill and Identifiable Intangible Assets, Net - Changes in Carrying Amount of Goodwill (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020
Changes in the carrying amount of goodwill
Balance at beginning of year	$ 464,392	$ 332,447
Acquisitions and purchase price adjustments (See Note 5)	62,572	131,945
Balance at end of period	526,964	464,392
Mechanical Services Segment
Changes in the carrying amount of goodwill
Balance at beginning of year	307,448	234,660
Acquisitions and purchase price adjustments (See Note 5)	8,089	72,788
Impact of segment reorganization	1,101
Balance at end of period	316,638	307,448
Electrical Services Industry
Changes in the carrying amount of goodwill
Balance at beginning of year	156,944	97,787
Acquisitions and purchase price adjustments (See Note 5)	54,483	59,157
Impact of segment reorganization	(1,101)
Balance at end of period	$ 210,326	$ 156,944

Goodwill and Identifiable Int_4

Goodwill and Identifiable Intangible Assets, Net - Identifiable Intangible Assets, Net (Details) $ in Thousands	Sep. 30, 2021USD ($)
Future amortization expense of identifiable intangible assets
2021 (remainder of the year)	$ 11,021
2022	36,968
2023	28,495
2024	27,145
2025	24,958
Thereafter	139,512
Total	$ 268,099

Goodwill and Identifiable Int_5

Goodwill and Identifiable Intangible Assets, Net - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2020	Sep. 30, 2021	Dec. 31, 2019
Goodwill
Goodwill	$ 464,392	$ 526,964	$ 332,447
Walker
Goodwill
Fair values in excess of carrying value (as a percent)	24.00%
Goodwill		$ 96,800

Debt Obligations (Details)

Debt Obligations (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Obligations
Finance lease liabilities (See Note 8)	$ 2,438
Total principal amount	272,938	$ 236,000
Less-unamortized debt issuance costs	(208)	(267)
Total debt, net of unamortized debt issuance costs	272,730	235,733
Less-current portion	5,958
Total long-term portion of debt, net	266,772	235,733
Revolving credit facility
Debt Obligations
Total principal amount	115,000	70,000
Term loan
Debt Obligations
Total principal amount	120,000	135,000
Notes to former owners
Debt Obligations
Outstanding balance	$ 35,500	$ 31,000

Debt Obligations - Other (Detai

Debt Obligations - Other (Details) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021USD ($)item	Jun. 30, 2021USD ($)	Mar. 31, 2021USD ($)	Sep. 30, 2020USD ($)	Jun. 30, 2020USD ($)	Mar. 31, 2020USD ($)	Sep. 30, 2021USD ($)item	Sep. 30, 2020USD ($)	Aug. 01, 2021USD ($)	Dec. 31, 2020USD ($)
Debt Obligations
Payments on term loan							$ 15,000	$ 15,000
Outstanding balance	$ 272,938						272,938			$ 236,000
Reconciliation of Credit Facility Adjusted EBITDA to net income
Net income	46,297	$ 32,965	$ 26,491	$ 50,088	$ 39,495	$ 17,716	105,753	107,299
Provision for income taxes	14,999			$ 8,279			33,553	30,100
Stock-based compensation							7,378	$ 5,949
Amteck
Other disclosures
Acquired debt									$ 785
Revolving credit facility
Debt Obligations
Borrowing capacity	450,000						450,000
Outstanding borrowings	115,000						115,000
Outstanding balance	115,000						115,000			70,000
Letters of credit amount outstanding	49,500						49,500
Credit available	$ 285,500						$ 285,500
Principal financial covenants
Number of interest rate options | item							2
Other disclosures
Weighted average interest rate (as a percent)	1.30%						1.30%
Revolving credit facility | Consolidated Total Indebtedness to Credit Facility Adjusted EBITDA: Less than 0.75
Debt Instrument, Interest Rate, Effective Percentage [Abstract]
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: 2.25 or greater
Debt Instrument, Interest Rate, Effective Percentage [Abstract]
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 | Base Rate | Consolidated Total Indebtedness to Credit Facility Adjusted EBITDA: Less than 0.75
Debt Instrument, Interest Rate, Effective Percentage [Abstract]
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
Debt Instrument, Interest Rate, Effective Percentage [Abstract]
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
Debt Instrument, Interest Rate, Effective Percentage [Abstract]
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
Debt Instrument, Interest Rate, Effective Percentage [Abstract]
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
Debt Instrument, Interest Rate, Effective Percentage [Abstract]
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
Debt Instrument, Interest Rate, Effective Percentage [Abstract]
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
Debt Instrument, Interest Rate, Effective Percentage [Abstract]
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
Debt Instrument, Interest Rate, Effective Percentage [Abstract]
Additional per annum interest margin (as a percent)							2.00%
Amended senior revolving credit facility
Debt Obligations
Borrowing capacity	$ 570,000						$ 570,000			600,000
Line of credit borrowing capacity accordion option	150,000						150,000
Payments on term loan							30,000
Term loan
Debt Obligations
Borrowing capacity	150,000						150,000
Outstanding balance	$ 120,000						$ 120,000			135,000
Other disclosures
Weighted average interest rate (as a percent)	1.30%						1.30%
Notes to former owners
Other disclosures
Cumulative number of companies acquired | item	6						6
Outstanding balance	$ 35,500						$ 35,500			$ 31,000
Promissory note | Walker
Other disclosures
Outstanding balance	$ 6,000						$ 6,000
Weighted average interest rate (as a percent)	4.00%						4.00%
Promissory note | TAS Energy
Other disclosures
Outstanding balance	$ 4,000						$ 4,000
Weighted average interest rate (as a percent)	3.50%						3.50%
Promissory note | T E C Industrial Construction and Maintenance
Other disclosures
Outstanding balance	$ 7,000						$ 7,000
Weighted average interest rate (as a percent)	2.50%						2.50%
Promissory note | Electrical Contractor North Carolina
Other disclosures
Outstanding balance	$ 5,000						$ 5,000
Weighted average interest rate (as a percent)	3.00%						3.00%
Promissory note | Amteck
Other disclosures
Outstanding balance	$ 10,000						$ 10,000
Weighted average interest rate (as a percent)	2.50%						2.50%
Promissory note | Mechanical Contractor Utah
Other disclosures
Outstanding balance	$ 3,500						$ 3,500
Weighted average interest rate (as a percent)	2.50%						2.50%
Letter of Credit
Debt Obligations
Borrowing capacity	$ 160,000						$ 160,000

Leases (Details)

Leases (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2021USD ($)Option	Sep. 30, 2020USD ($)	Dec. 31, 2020
Lessee, Lease, Description [Line Items]
Finance lease liability	$ 2,438		$ 2,438
Variable lease expense and short-term lease expenses			$ 5,700	$ 6,100
Weighted average discount rate for operating leases	4.10%		4.10%		4.20%
Operating lease expense	$ 7,700	$ 8,000	$ 22,100	21,400
Weighted average remaining lease term	7 years 7 months 6 days		7 years 7 months 6 days		7 years 6 months
Rent paid to related parties	$ 1,100	$ 2,000	$ 3,600	$ 4,500
Existence of option to extend			true
Minimum
Lessee, Lease, Description [Line Items]
Lease term for operating lease	3 years		3 years
Number of options to renew | Option			1
Maximum
Lessee, Lease, Description [Line Items]
Lease term for operating lease	10 years		10 years

Leases - Summary of Lease Asset

Leases - Summary of Lease Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Summary of lease asset and liabilities
Lease right-of-use assets	$ 104,336	$ 94,727
Lease liabilities:
Other current liabilities	$ 17,499	$ 16,586
Operating Lease, Liability, Current, Statement of Financial Position [Extensible List]	Other Liabilities, Current	Other Liabilities, Current
Long-term lease liabilities	$ 88,923	$ 80,576
Total lease liabilities	$ 106,422	$ 97,162

Leases - Maturities of Lease Li

Leases - Maturities of Lease Liabilities (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Maturities of lease liabilities:
2021 (excluding the nine months ended September 30, 2021)	$ 5,668		$ 5,668
2022	20,771		20,771
2023	18,101		18,101
2024	16,066		16,066
2025	15,180		15,180
Thereafter	49,092		49,092
Total Lease Payments	124,878		124,878
Less-Present Value Discount	(18,456)		(18,456)
Present Value of Lease Liabilities	106,422		106,422		$ 97,162
Supplemental information related to leases:
Cash paid for amounts included in the measurement of lease liabilities	5,597	$ 5,362	16,297	$ 15,140
Lease right-of-use assets obtained in exchange for lease liabilities	$ 13,154	$ 1,880	$ 22,513	$ 24,080

Commitments and Contingencies -

Commitments and Contingencies - Other and Bonds (Details) $ in Millions	9 Months Ended
	Sep. 30, 2021USD ($)item
Obligation to perform subcontract work | Pending Litigation
Commitments and Contingencies
Letter of intent under dispute | item	2
Damages claimed	$ 9
Obligation to perform subcontract work | Customer | Pending Litigation
Commitments and Contingencies
Damages claimed	$ 15
Surety | Minimum
Surety
Percentage of business which has required bonds	15.00%
Surety | Maximum
Surety
Percentage of business which has required bonds	25.00%

Stockholders' Equity - Incentiv

Stockholders' Equity - Incentive and Other (Details) - USD ($) $ / shares in Units, $ in Thousands, shares in Millions	9 Months Ended		174 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Dec. 08, 2020	Mar. 29, 2007
Share Repurchase Program
Share repurchase	$ 25,531	$ 18,777
Stock Repurchase Program 2007
Share Repurchase Program
Number of shares of outstanding common stock authorized to be acquired under a stock repurchase program	10.3		10.3		1
Additional number of shares authorized for repurchase				0.7
Share repurchase (in shares)			9.7
Average price (in dollars per share)	$ 73.69		$ 21.57
Share repurchase	$ 25,500
Maximum | Stock Repurchase Program 2007
Share Repurchase Program
Share repurchase (in shares)	0.3

Stockholders' Equity - Number o

Stockholders' Equity - Number of Shares (Details) - shares shares in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Reconciliation of the number of shares outstanding with the number of shares used in computing basic and diluted earnings per share
Common shares outstanding, end of period	36,042	36,400	36,042	36,400
Effect of using weighted average common shares outstanding	254	157	286	204
Shares used in computing earnings per share-basic	36,296	36,557	36,328	36,604
Effect of shares issuable under stock option plans based on the treasury stock method	73	125	100	124
Effect of restricted and contingently issuable shares	65	68	72	69
Shares used in computing earnings per share-diluted	36,434	36,750	36,500	36,797

Segment Information (Details)

Segment Information (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021USD ($)	Sep. 30, 2020USD ($)	Sep. 30, 2021USD ($)segment	Sep. 30, 2020USD ($)
Segment Information
Number of reportable segments | segment			2
Revenue	$ 833,896	$ 714,099	$ 2,217,552	$ 2,157,698
Gross Profit	159,212	147,196	409,136	409,984
Operating | Mechanical Services Segment
Segment Information
Revenue	690,680	625,660	1,868,096	1,823,117
Gross Profit	138,618	139,845	359,151	386,537
Operating | Electrical Services Industry
Segment Information
Revenue	143,216	88,439	349,456	334,581
Gross Profit	$ 20,594	$ 7,351	$ 49,985	$ 23,447