Tetra Tech (TTEK) 10-Q 4 May 18 2018 Q2 Quarterly report Financial data

Document and Entity Information

Document and Entity Information - shares	6 Months Ended
	Apr. 01, 2018	Apr. 30, 2018
Document and Entity Information
Entity Registrant Name	TETRA TECH INC
Entity Central Index Key	831,641
Document Type	10-Q
Document Period End Date	Apr. 1, 2018
Amendment Flag	false
Current Fiscal Year End Date	--09-30
Entity Current Reporting Status	Yes
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		55,615,925
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	Q2

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Apr. 01, 2018	Oct. 01, 2017
Current assets:
Cash and cash equivalents	$ 203,177	$ 189,975
Accounts receivable – net	863,219	788,767
Prepaid expenses and other current assets	65,102	49,969
Income taxes receivable	18,406	13,312
Total current assets	1,149,904	1,042,023
Property and equipment – net	55,495	56,835
Investments in unconsolidated joint ventures	3,167	2,700
Goodwill	809,690	740,886
Intangible assets – net	20,847	26,688
Deferred tax assets	8,588	1,763
Other long-term assets	35,102	31,850
Total assets	2,082,793	1,902,745
Current liabilities:
Accounts payable	155,257	177,638
Accrued compensation	140,426	143,408
Billings in excess of costs on uncompleted contracts	144,327	117,499
Current portion of long-term debt	15,466	15,588
Current contingent earn-out liabilities	11,669	2,024
Other current liabilities	92,863	81,511
Total current liabilities	560,008	537,668
Deferred tax liabilities	45,740	43,781
Long-term debt	463,544	341,283
Long-term contingent earn-out liabilities	18,443	414
Other long-term liabilities	54,830	50,975
Commitments and contingencies (Note 14)
Equity:
Preferred stock - authorized, 2,000 shares of $0.01 par value; no shares issued and outstanding at April 1, 2018 and October 1, 2017	0	0
Common stock - authorized, 150,000 shares of $0.01 par value; issued and outstanding, 55,749 and 55,873 shares at April 1, 2018 and October 1, 2017, respectively	557	559
Additional paid-in capital	161,387	193,835
Accumulated other comprehensive loss	(118,044)	(98,500)
Retained earnings	896,146	832,559
Tetra Tech stockholders’ equity	940,046	928,453
Noncontrolling interests	182	171
Total stockholders' equity	940,228	928,624
Total liabilities and stockholders' equity	$ 2,082,793	$ 1,902,745

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Apr. 01, 2018	Oct. 01, 2017
Statement of Financial Position [Abstract]
Preferred stock, authorized shares (in shares)	2,000,000	2,000,000
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, authorized shares (in shares)	150,000,000	150,000,000
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares issued (in shares)	55,749,000	55,873,000
Common stock, shares outstanding (in shares)	55,749,000	55,873,000

Consolidated Statements of Inco

Consolidated Statements of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Apr. 01, 2018	Apr. 02, 2017	Apr. 01, 2018	Apr. 02, 2017
Income Statement [Abstract]
Revenue	$ 700,262	$ 663,781	$ 1,460,010	$ 1,332,632
Subcontractor costs	(167,469)	(151,827)	(382,370)	(331,127)
Other costs of revenue	(441,368)	(430,952)	(892,070)	(839,141)
Gross profit	91,425	81,002	185,570	162,364
Selling, general and administrative expenses	(46,791)	(45,195)	(92,347)	(86,702)
Contingent consideration – fair value adjustments	(1,918)	7,149	(1,918)	7,149
Income from operations	42,716	42,956	91,305	82,811
Interest expense	(4,092)	(3,099)	(7,252)	(6,007)
Income before income tax expense	38,624	39,857	84,053	76,804
Income tax expense	(9,877)	(12,990)	(9,254)	(23,348)
Net income	28,747	26,867	74,799	53,456
Net income attributable to noncontrolling interests	(22)	(5)	(40)	(32)
Net income attributable to Tetra Tech	$ 28,725	$ 26,862	$ 74,759	$ 53,424
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.51	$ 0.47	$ 1.34	$ 0.93
Diluted (in dollars per share)	$ 0.51	$ 0.46	$ 1.32	$ 0.92
Weighted-average common shares outstanding:
Basic (in shares)	55,841	57,270	55,900	57,171
Diluted (in shares)	56,673	58,270	56,825	58,194
Cash dividends paid per share (in dollars per share)	$ 0.1	$ 0.09	$ 0.2	$ 0.18

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Apr. 01, 2018	Apr. 02, 2017	Apr. 01, 2018	Apr. 02, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 28,747	$ 26,867	$ 74,799	$ 53,456
Foreign currency translation adjustments	(16,121)	9,328	(19,590)	(6,671)
Gain (loss) on cash flow hedge valuations	(27)	452	39	1,448
Other comprehensive income (loss), net of tax	(16,148)	9,780	(19,551)	(5,223)
Comprehensive income, net of tax	12,599	36,647	55,248	48,233
Net income attributable to noncontrolling interests	(22)	(5)	(40)	(32)
Foreign currency translation adjustments	5	(5)	7	(188)
Comprehensive income attributable to noncontrolling interests	(17)	(10)	(33)	(220)
Comprehensive income attributable to Tetra Tech, net of tax	$ 12,582	$ 36,637	$ 55,215	$ 48,013

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Apr. 01, 2018	Apr. 02, 2017
Cash flows from operating activities:
Net income	$ 74,799	$ 53,456
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization	20,312	23,234
Equity in income of unconsolidated joint ventures	(2,032)	(2,489)
Distributions of earnings from unconsolidated joint ventures	1,470	2,301
Non-cash stock compensation	8,705	6,610
Deferred income taxes	(10,100)	19,851
Provision for doubtful accounts	2,390	(2,028)
Fair value adjustments to contingent consideration	1,918	(7,149)
Gain on disposal of property and equipment	(1,205)	(266)
Changes in operating assets and liabilities, net of effects of business acquisitions:
Accounts receivable	(42,912)	2,631
Prepaid expenses and other assets	(15,528)	(7,587)
Accounts payable	(26,460)	(15,587)
Accrued compensation	(11,453)	(12,888)
Billings in excess of costs on uncompleted contracts	15,367	25,139
Other liabilities	716	(5,021)
Income taxes receivable/payable	(1,213)	(29,843)
Net cash provided by operating activities	14,774	50,364
Cash flows from investing activities:
Payments for business acquisitions, net of cash acquired	(64,451)	(8,039)
Capital expenditures	(4,565)	(3,896)
Proceeds from sale of property and equipment	1,651	303
Net cash used in investing activities	(67,365)	(11,632)
Cash flows from financing activities:
Proceeds from borrowings	180,026	133,596
Payments on long-term debt	(62,109)	(137,849)
Repurchases of common stock	(50,000)	(20,000)
Net proceeds from issuance of common stock	11,945	7,661
Dividends paid	(11,172)	(10,301)
Payments of contingent earn-out liabilities	(854)	0
Net cash provided by (used in) financing activities	67,836	(26,893)
Effect of exchange rate changes on cash	(2,043)	(958)
Net increase in cash and cash equivalents	13,202	10,881
Cash and cash equivalents at beginning of period	189,975	160,459
Cash and cash equivalents at end of period	203,177	171,340
Cash paid during the period for:
Interest	6,895	5,970
Income taxes, net of refunds received of $0.3 million and $0.6 million	$ 20,151	$ 32,995

Consolidated Statements of Cas7

Consolidated Statements of Cash Flows (Parenthetical) - USD ($) $ in Millions	6 Months Ended
	Apr. 01, 2018	Apr. 02, 2017
Statement of Cash Flows [Abstract]
Income tax refunds	$ 0.3	$ 0.6

Basis of Presentation

Basis of Presentation	6 Months Ended
	Apr. 01, 2018
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	Basis of Presentation The accompanying unaudited interim consolidated financial statements and related notes of Tetra Tech, Inc. (“we,” “us” , “our” or "Tetra Tech") have been prepared in accordance with generally accepted accounting principles in the United States of America (“GAAP”) for interim financial information and with the instructions to Form 10-Q and Rule 10-01 of Regulation S-X. They do not include all of the information and footnotes required by GAAP for complete financial statements and, therefore, should be read in conjunction with the audited consolidated financial statements and related notes contained in our Annual Report on Form 10-K for the fiscal year ended October 1, 2017 . These financial statements reflect all normal recurring adjustments that are considered necessary for a fair statement of our financial position, results of operations and cash flows for the interim periods presented. The results of operations and cash flows for any interim period are not necessarily indicative of results for the full year or for future years. Beginning in fiscal 2018 , we aligned our operations to better serve our clients and markets, resulting in two renamed reportable segments. Our Government Services Group (“GSG”) reportable segment primarily includes activities with U.S. government clients (federal, state and local) and all activities with development agencies worldwide. Our Commercial/International Services Group (“CIG”) reportable segment primarily includes activities with U.S. commercial clients and all international activities other than work for development agencies. This alignment allows us to capitalize on our growing market opportunities and enhance the development of high-end consulting and technical solutions to meet our growing client demand. We continue to report the results of the wind-down of our non-core construction activities in the Remediation and Construction Management (“RCM”) segment. Prior year amounts for reportable segments have been revised to conform to the current-year presentation.

Accounts Receivable - Net and R

Accounts Receivable - Net and Revenue Recognition	6 Months Ended
	Apr. 01, 2018
Accounts Receivable - Net and Revenue Recognition [Abstract]
Accounts Receivable - Net and Revenue Recognition	Accounts Receivable – Net and Revenue Recognition Net accounts receivable and billings in excess of costs on uncompleted contracts consisted of the following: April 1, October 1, (in thousands) Billed $ 461,781 $ 376,287 Unbilled 413,517 404,899 Contract retentions 22,019 39,840 Total accounts receivable – gross 897,317 821,026 Allowance for doubtful accounts (34,098 ) (32,259 ) Total accounts receivable – net $ 863,219 $ 788,767 Billings in excess of costs on uncompleted contracts $ 144,327 $ 117,499 Billed accounts receivable represent amounts billed to clients that have not been collected. Unbilled accounts receivable represent revenue recognized but not yet billed pursuant to contract terms or billed after the period end date. Except for amounts related to claims as discussed below, most of our unbilled receivables at April 1, 2018 are expected to be billed and collected within 12 months. Contract retentions represent amounts withheld by clients until certain conditions are met or the project is completed, which may be several months or years. The allowance for doubtful accounts represents amounts that are expected to become uncollectible or unrealizable in the future. We determine an estimated allowance for uncollectible accounts based on management's consideration of trends in the actual and forecasted credit quality of our clients, including delinquency and payment history; type of client, such as a government agency or a commercial sector client; and general economic and particular industry conditions that may affect a client's ability to pay. Billings in excess of costs on uncompleted contracts represent the amount of cash collected from clients and billings to clients on contracts in advance of revenue recognized. The majority of billings in excess of costs on uncompleted contracts will be earned within 12 months. Once contract performance is underway, we may experience changes in conditions, client requirements, specifications, designs, materials and expectations regarding the period of performance. Such changes result in change orders and may be initiated by us or by our clients. In many cases, agreement with the client as to the terms of change orders is reached prior to work commencing; however, sometimes circumstances require that work progress without a definitive client agreement. Unapproved change orders constitute claims in excess of agreed contract prices that we seek to collect from our clients for delays, errors in specifications and designs, contract terminations, or other causes of unanticipated additional costs. Revenue on claims is recognized when contract costs related to claims have been incurred and when their addition to contract value can be reliably estimated. This can lead to a situation in which costs are recognized in one period and revenue is recognized in a subsequent period, such as when client agreement is obtained or a claims resolution occurs. Total accounts receivable at April 1, 2018 and October 1, 2017 included $65 million and $59 million , respectively, related to claims, including requests for equitable adjustment, on contracts that provide for price redetermination. We regularly evaluate all unsettled claim amounts and record appropriate adjustments to operating earnings when it is probable that the claim will result in a different contract value than the amount previously estimated. We recorded no material gains or losses related to claims in the first half of fiscal 2018 . In the first half of fiscal 2017 (all in the second quarter), we recognized a reduction of revenue of $4.5 million and a related loss in operating income of $3.2 million in our RCM segment. Billed accounts receivable related to U.S. federal government contracts were $ 89.3 million and $ 45.4 million at April 1, 2018 and October 1, 2017 , respectively. U.S. federal government contracts unbilled receivables were $ 104.5 million and $ 109.7 million at April 1, 2018 and October 1, 2017 , respectively. Other than the U.S. federal government, no single client accounted for more than 10% of our accounts receivable at April 1, 2018 and October 1, 2017 . We recognize our revenue from contracts using the percentage-of-completion method, primarily utilizing the cost-to-cost approach, to estimate the progress towards completion in order to determine the amount of revenue and profit to recognize. Changes in those estimates could result in the recognition of cumulative catch-up adjustments to the contract’s inception-to-date revenue, costs and profit in the period in which such changes are made. As a result, we recognized net unfavorable operating income adjustments of $0.7 million and $1.4 million during the second quarter and first half of fiscal 2018 , respectively, in the CIG segment. We recognized net unfavorable operating income adjustments of $4.0 million (all in the RCM segment) during the second quarter of fiscal 2017 , and $8.0 million ( $2.3 million in the CIG segment and $5.7 million in the RCM segment) during the first half of fiscal 2017 . Changes in revenue and cost estimates could also result in a projected loss that would be recorded immediately in earnings. As of April 1, 2018 and October 1, 2017 , our consolidated balance sheets included liabilities for anticipated losses of $4.4 million and $8.1 million , respectively. The estimated cost to complete the related contracts as of April 1, 2018 was $4.6 million .

Mergers and Acquisitions

Mergers and Acquisitions	6 Months Ended
	Apr. 01, 2018
Business Combinations [Abstract]
Mergers and Acquisitions	Mergers and Acquisitions In the second quarter of fiscal 2018, we completed the acquisition of Norman Disney & Young (“NDY”), a leader in sustainable infrastructure engineering design. NDY is an Australian-based global engineering design firm with more than 700 professionals operating in offices throughout Australia, the Asia-Pacific region, the United Kingdom, and Canada and is part of our CIG segment. The fair value of the purchase price for NDY was $56.1 million . This amount is comprised of $46.3 million of initial cash payments made to the sellers, $1.6 million held in escrow and $0.6 million in accruals included in current liabilities for pending resolution of the closing balance sheet, and $7.6 million for the estimated fair value of contingent earn-out obligations, with a maximum amount of $20.2 million , based upon the achievement of specified operating income targets in each of the three years following the acquisition. In the first quarter of fiscal 2017, we acquired Glumac, headquartered in Portland, Oregon. Glumac is a leader in sustainable infrastructure design with more than 300 employees and is part of our GSG segment. The fair value of the purchase price for Glumac was $38.4 million . This amount is comprised of $19.0 million of initial cash payments made to the sellers, $1.0 million held in escrow and included in current liabilities for pending resolution of the closing balance sheet, and $18.4 million for the estimated fair value of contingent earn-out obligations, with a maximum of $20.0 million payable, based upon the achievement of specified operating income targets in each of the three years following the acquisition. In the second quarter of fiscal 2017, we acquired Eco Logical Australia (“ELA”), headquartered in Sydney, Australia. ELA is a multi-disciplinary consulting firm with over 160 staff that provides innovative, high-end environmental and ecological services, and is part of our CIG segment. The fair value of the purchase price for ELA was $9.9 million . This amount consists of $8.3 million of cash payments made to the sellers and $1.6 million for the estimated fair value of contingent earn-out obligations, with a maximum of $1.7 million payable, based upon the achievement of specified operating income targets in each of the two years following the acquisition. Goodwill additions resulting from the above business combinations are primarily attributable to the existing workforce of the acquired companies and the synergies expected to arise after the acquisitions. The goodwill addition related to the fiscal 2018 acquisitions primarily represent the value of a workforce with distinct expertise in the sustainable infrastructure design market. The goodwill addition related to the fiscal 2017 acquisition primarily represents the value of a workforce with distinct expertise in the environmental and ecological markets. In addition, these acquired capabilities, when combined with our existing global consulting and engineering business, result in opportunities that allow us to provide services under contracts that could not have been pursued individually by either us or the acquired companies. The results of these acquisitions were included in our consolidated financial statements from their respective closing dates. These acquisitions were not considered material to our consolidated financial statements. As a result, no pro forma information has been provided. Backlog, client relations and trade names intangible assets include the fair value of existing contracts and the underlying customer relationships with lives ranging from 1 to 10 years, and trade names with lives ranging from 3 to 5 years. For detailed information regarding our intangible assets, see Note 4 , “ Goodwill and Intangible Assets ”. Most of our acquisition agreements include contingent earn-out agreements, which are generally based on the achievement of future operating income thresholds. The contingent earn-out arrangements are based on our valuations of the acquired companies, and reduce the risk of overpaying for acquisitions if the projected financial results are not achieved. The fair values of any earn-out arrangements are included as part of the purchase price of the acquired companies on their respective acquisition dates. For each transaction, we estimate the fair value of contingent earn-out payments as part of the initial purchase price and record the estimated fair value of contingent consideration as a liability in “Current contingent earn-out liabilities” and “Long-term contingent earn-out liabilities” on our consolidated balance sheets. We consider several factors when determining that contingent earn-out liabilities are part of the purchase price, including the following: (1) the valuation of our acquisitions is not supported solely by the initial consideration paid, and the contingent earn-out formula is a critical and material component of the valuation approach to determining the purchase price; and (2) the former owners of acquired companies that remain as key employees receive compensation other than contingent earn-out payments at a reasonable level compared with the compensation of our other key employees. The contingent earn-out payments are not affected by employment termination. We measure our contingent earn-out liabilities at fair value on a recurring basis using significant unobservable inputs classified within Level 3 of the fair value hierarchy. We use a probability-weighted discounted income approach as a valuation technique to convert future estimated cash flows to a single present value amount. The significant unobservable inputs used in the fair value measurements are operating income projections over the earn-out period (generally two or three years), and the probability outcome percentages we assign to each scenario. Significant increases or decreases to either of these inputs in isolation would result in a significantly higher or lower liability, with a higher liability capped by the contractual maximum of the contingent earn-out obligation. Ultimately, the liability will be equivalent to the amount paid, and the difference between the fair value estimate and amount paid will be recorded in earnings. The amount paid that is less than or equal to the contingent earn-out liability on the acquisition date is reflected as cash used in financing activities in our consolidated statements of cash flows. Any amount paid in excess of the contingent earn-out liability on the acquisition date is reflected as cash used in operating activities. We review and re-assess the estimated fair value of contingent consideration on a quarterly basis, and the updated fair value could differ materially from the initial estimates. Changes in the estimated fair value of our contingent earn-out liabilities related to the time component of the present value calculation are reported in interest expense. Adjustments to the estimated fair value related to changes in all other unobservable inputs are reported in operating income. During the first half of fiscal 2018 (all in the second quarter), we recorded increases in our contingent earn-out liabilities related to ELA and Cornerstone Environmental Group ("CEG"), and reported related losses in operating income totaling $1.9 million . During the first half of fiscal 2017 (all in the second quarter), we recorded decreases in our contingent earn-out liabilities related to INDUS Corporation ("INDUS") and CEG, and reported related gains in operating income totaling $7.1 million ( $5.0 million for INDUS and $2.1 million for CEG). The acquisition agreement for INDUS included a contingent earn-out agreement based on the achievement of operating income thresholds in each of the first two years beginning on the acquisition date, which was in the second quarter of fiscal 2016. The maximum earn-out obligation over the two -year earn-out period was $8.0 million ( $4.0 million in each year). These amounts could be earned on a pro-rata basis starting at 50% of the earn-out maximum for operating income within a predetermined range in each year. INDUS was required to meet a minimum operating income threshold in each year to earn any contingent consideration. These minimum thresholds were $3.2 million and $3.6 million in years one and two, respectively. In order to earn the maximum contingent consideration, INDUS needed to generate operating income of $3.6 million in year one and $4.0 million in year two. The determination of the fair value of the purchase price for INDUS on the acquisition date included our estimate of the fair value of the related contingent earn-out obligation. The initial valuation was primarily based on probability-weighted internal estimates of INDUS' operating income during each earn-out period. As a result of these estimates, we calculated an initial fair value at the acquisition date of INDUS' contingent earn-out liability of $4.7 million in the second quarter of fiscal 2016. This amount had increased to $4.9 million at the end of fiscal 2016 due to the passage of time for the present value calculation. In determining that INDUS would earn 59% of the maximum potential earn-out, we considered several factors including INDUS' recent historical revenue and operating income levels and growth rates. We also considered the recent trend in INDUS' backlog level. INDUS' annual financial performance in the first earn-out period was below our original expectation at the acquisition date. As a result, in the second quarter of fiscal 2017, we evaluated our estimate of INDUS' contingent consideration liability for both earn-out periods. This assessment included a review of INDUS' financial results in the first earn-out period, the status of ongoing projects in INDUS' backlog, and the inventory of prospective new contract awards. As a result of this assessment, we concluded that INDUS' operating income in both the first and second earn-out periods would be lower than the minimum requirements of $3.2 million and $3.6 million , respectively, to earn any contingent consideration. Accordingly, in the second quarter of fiscal 2017, we reduced the INDUS contingent earn-out liability to $0 , which resulted in a gain of $5.0 million . At April 1, 2018 , there was a total potential maximum of $43.4 million of outstanding contingent consideration related to acquisitions. Of this amount, $ 30.1 million was estimated as the fair value and accrued on our consolidated balance sheet.

Goodwill and Intangible Assets

Goodwill and Intangible Assets	6 Months Ended
	Apr. 01, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Intangible Assets	Goodwill and Intangible Assets The following table summarizes the changes in the carrying value of goodwill: GSG CIG Total (in thousands) Balance at October 1, 2017 $ 361,761 $ 379,125 $ 740,886 Goodwill additions 26,726 54,275 81,001 Foreign exchange impact (2,138 ) (10,059 ) (12,197 ) Balance at April 1, 2018 $ 386,349 $ 423,341 $ 809,690 During the first half of fiscal 2018, we recorded goodwill additions as a result of our recent acquisitions. The purchase price allocations for these acquisitions are preliminary, and subject to adjustment based upon the final determination of the net assets acquired and information to perform the final valuation. Foreign exchange impact relates to our foreign subsidiaries with functional currencies that are different than our reporting currency. The gross amounts of goodwill for GSG were $404.0 million and $379.5 million at April 1, 2018 and October 1, 2017 , respectively, excluding $17.7 million of accumulated impairment. The gross amounts of goodwill for CIG were $521.3 million and $477.0 million at April 1, 2018 and October 1, 2017 , respectively, excluding $97.9 million of accumulated impairment. We perform our annual goodwill impairment review at the beginning of our fiscal fourth quarter. Our most recent annual review at July 3, 2017 (i.e. the first day of our fourth quarter in fiscal 2017 ) indicated that we had no impairment of goodwill, and all of our reporting units had estimated fair values that were in excess of their carrying values, including goodwill. We regularly evaluate whether events and circumstances have occurred that may indicate a potential change in the recoverability of goodwill. We perform interim goodwill impairment reviews between our annual reviews if certain events and circumstances have occurred, such as a deterioration in general economic conditions; an increase in the competitive environment; a change in management, key personnel, strategy, or customers; negative or declining cash flows; or a decline in actual or planned revenue or earnings compared with actual and projected results of relevant prior periods. The reorganization of our core operations, described in Note 1 , “ Basis of Presentation ” and Note 10 , “ Reportable Segments ”, also impacted the definition of our reporting units for goodwill impairment testing. As a result, on October 2, 2017, we performed impairment testing for goodwill under our new segment structure and determined that the estimated fair value of each new reporting unit exceeded its corresponding carrying amount including recorded goodwill. Although we believe that our estimates of fair value for our reporting units are reasonable, if financial performance for our reporting units falls significantly below our expectations or market prices for similar businesses decline, the goodwill for our reporting units could become impaired. We estimate the fair value of all reporting units with a goodwill balance based on a comparison and weighting of the income approach (weighted 70% ), specifically the discounted cash flow method, and the market approach (weighted 30% ), which estimates the fair value of our reporting units based upon comparable market prices and recent transactions and also validates the reasonableness of the multiples from the income approach. The resulting fair value is most sensitive to the assumptions we use in our discounted cash flow analysis. The assumptions that have the most significant impact on the fair value calculation are the reporting unit’s revenue growth rate and operating profit margin, and the discount rate used to convert future estimated cash flows to a single present value amount. The gross amount and accumulated amortization of our acquired identifiable intangible assets with finite useful lives included in “Intangible assets - net” on our consolidated balance sheets, were as follows: April 1, 2018 October 1, 2017 Weighted- Average Remaining Life (in Years) Gross Amount Accumulated Amortization Gross Amount Accumulated Amortization ($ in thousands) Non-compete agreements 0.0 $ 83 $ (83 ) $ 495 $ (493 ) Client relations 2.7 54,643 (43,119 ) 90,297 (75,074 ) Backlog 0.8 23,184 (16,443 ) 21,518 (13,301 ) Trade names 2.8 5,248 (2,666 ) 6,685 (3,439 ) Total $ 83,158 $ (62,311 ) $ 118,995 $ (92,307 ) The gross amount and accumulated amortization for client relations decreased due to the full amortization of assets as of fiscal 2017 year-end. Amortization expense for the three and six months ended April 1, 2018 was $ 5.0 million and $9.7 million , respectively, compared to $ 5.9 million and $11.8 million for the prior-year periods. Estimated amortization expense for the remainder of fiscal 2018 and succeeding years is as follows: Amount (in thousands) 2018 $ 8,300 2019 7,471 2020 2,786 2021 1,650 2022 411 Beyond 229 Total $ 20,847

Property and Equipment

Property and Equipment	6 Months Ended
	Apr. 01, 2018
Property, Plant and Equipment [Abstract]
Property and Equipment	Property and Equipment Property and equipment consisted of the following: April 1, October 1, (in thousands) Equipment, furniture and fixtures 147,316 150,026 Leasehold improvements 31,066 27,689 Land and buildings $ 3,746 $ 3,680 Total property and equipment 182,128 181,395 Accumulated depreciation (126,633 ) (124,560 ) Property and equipment, net $ 55,495 $ 56,835 The depreciation expense related to property and equipment was $ 5.1 million and $10.3 million for the three and six months ended April 1, 2018 , respectively, compared to $ 5.8 million and 11.0 million for the prior-year periods.

Stock Repurchase and Dividends

Stock Repurchase and Dividends	6 Months Ended
	Apr. 01, 2018
Stock Repurchase and Dividends
Stock Repurchase and Dividends	Stock Repurchase and Dividends On November 7, 2016, the Board of Directors authorized a new stock repurchase program under which we could repurchase up to $200 million of our common stock. In the first half of fiscal 2018 , we repurchased through open market purchases under this program a total of 1,020,033 shares at an average price of $ 49.02 for a total cost of $ 50.0 million. On November 7, 2017, the Board of Directors declared a quarterly cash dividend of $0.10 per share payable on December 15, 2017 to stockholders of record as of the close of business on November 30, 2017. On January 29, 2018, the Board of Directors declared a quarterly cash dividend of $0.10 per share payable on March 2, 2018 to stockholders of record as of the close of business on February 14, 2018. Dividends totaling $ 11.2 million and $ 10.3 million were paid in the first half of fiscal 2018 and 2017 , respectively. Subsequent Event. On April 30, 2018, the Board of Directors declared a quarterly cash dividend of $0.12 per share payable on June 1, 2018 to stockholders of record as of the close of business on May 16, 2018.

Stockholders' Equity and Stock

Stockholders' Equity and Stock Compensation Plans	6 Months Ended
	Apr. 01, 2018
Stockholders' Equity Note [Abstract]
Stockholders' Equity and Stock Compensation Plans	Stockholders’ Equity and Stock Compensation Plans We recognize the fair value of our stock-based compensation awards as compensation expense on a straight-line basis over the requisite service period in which the award vests. Stock-based compensation expense for the three and six months ended April 1, 2018 was $ 4.7 million and $8.7 million , respectively, compared to $ 3.4 million and $6.6 million for the same periods last year. The majority of these amounts were included in “Selling, general and administrative (“SG&A”) expenses” in our consolidated statements of income. There were no material stock compensation awards in the second quarter of fiscal 2018 . In the six months ended April 1, 2018 , we granted 170,222 stock options with an exercise price of $47.95 per share and an estimated weighted-average fair value of $14.79 per share to our non-employee directors and executive officers. The executive officer options vest over a four -year period, and the non-employee director options vest after one year. In addition, we awarded 98,599 performance shares units (“PSUs”) to our non-employee directors and executive officers at a fair value of $56.10 per share on the award date. All of the PSUs are performance-based and vest, if at all, after the conclusion of the three -year performance period. The number of PSUs that ultimately vest is based 50% on the growth in our diluted earnings per share and 50% on our total shareholder return relative to a peer group of companies and a stock market index over the vesting period. Additionally, we awarded 189,651 restricted stock units (“RSUs”) to our non-employee directors, executive officers and employees at the fair value of $48.01 per share on the award date. All of the executive officer and employee RSUs have time-based vesting over a four -year period, and the non-employee director RSUs vest after one year.

Earnings per Share ("EPS")

Earnings per Share ("EPS")	6 Months Ended
	Apr. 01, 2018
Earnings Per Share [Abstract]
Earnings per Share (EPS)	Earnings per Share (“EPS”) Basic EPS is computed by dividing net income available to common stockholders by the weighted-average number of common shares outstanding, less unvested restricted stock for the period. Diluted EPS is computed by dividing net income by the weighted-average number of common shares outstanding and dilutive potential common shares for the period. Potential common shares include the weighted-average dilutive effects of outstanding stock options and unvested restricted stock using the treasury stock method. The following table sets forth the number of weighted-average shares used to compute basic and diluted EPS: Three Months Ended Six Months Ended April 1, April 2, April 1, April 2, (in thousands, except per share data) Net income attributable to Tetra Tech $ 28,725 $ 26,862 $ 74,759 $ 53,424 Weighted-average common shares outstanding – basic 55,841 57,270 55,900 57,171 Effect of dilutive stock options and unvested restricted stock 832 1,000 925 1,023 Weighted-average common stock outstanding – diluted 56,673 58,270 56,825 58,194 Earnings per share attributable to Tetra Tech: Basic $ 0.51 $ 0.47 $ 1.34 $ 0.93 Diluted $ 0.51 $ 0.46 $ 1.32 $ 0.92 For the three and six months of fiscal 2018 and 2017, no options were excluded from the calculation of dilutive potential common shares.

Income Taxes

Income Taxes	6 Months Ended
	Apr. 01, 2018
Income Tax Disclosure [Abstract]
Income Taxes	Income Taxes The effective tax rates for the first half of fiscal 2018 and 2017 were 11.0% and 30.4% , respectively. The fiscal 2018 tax rate reflects the impact of the comprehensive tax legislation enacted by the U.S. government on December 22, 2017, which is commonly referred to as the Tax Cuts and Jobs Act (“TCJA”). The TCJA significantly revised the U.S. corporate income tax regime by, among other things, lowering the U.S. corporate tax rate from 35% to 21% effective January 1, 2018, while also repealing the deduction for domestic production activities, limiting the deductibility of certain executive compensation, implementing a modified territorial tax system, and imposing a one-time repatriation tax on deemed repatriated earnings of foreign subsidiaries. As we have a September 30 fiscal year-end, our U.S. federal corporate income tax rate will be blended in fiscal 2018, resulting in a statutory federal rate of approximately 24.5% (3 months at 35% and 9 months at 21%), and will be 21% for subsequent fiscal years. GAAP requires that the impact of tax legislation be recognized in the period in which the tax law was enacted. As a result of the TCJA, we reduced our deferred tax liabilities and recorded a one-time deferred tax benefit of approximately $10.1 million in the first quarter of fiscal 2018 to reflect our estimate of temporary differences in the United States that will be recovered or settled in fiscal 2018 based on the 24.5% blended corporate tax rate or based on the 21% tax rate in fiscal 2019 and beyond versus the previous enacted 35% corporate tax rate. Excluding this tax benefit, our effective tax rate in the first half of fiscal 2018 was 23.0% . The TCJA also imposes a one-time transition tax on deemed repatriation of historical earnings of foreign subsidiaries. We analyzed this provision of the TCJA and our related foreign earnings accumulated under legacy tax laws during the half of fiscal 2018. Based on our preliminary analysis of tax earnings and profits and tax deficits at the prescribed measurement dates, we have a cumulative net tax deficit and do not believe we have any tax liability related to this tax. The one-time revaluation of our deferred tax liabilities and our estimate of the one-time transition tax on foreign earnings are both preliminary and subject to adjustment as we refine the information necessary to record the final values. The provisional amounts incorporate assumptions made based on our current interpretation of the TCJA and may change as we receive additional clarification on the implementation guidance. Additionally, in order to complete the valuation of our deferred tax liabilities, additional information related to the timing of the recovery or settlement of our deferred tax assets and liabilities and the effective tax rates (including state tax rates) that will apply needs to be obtained and analyzed. Similarly, information related to the computation of our foreign earnings and profits subject to the one-time transition tax requires further analysis before we make a final determination that we have no related liability. The U.S. Securities and Exchange Commission (“SEC”) has issued rules that would allow for a measurement period of up to one year after the enactment date of the TCJA to finalize the recording of the related tax impacts. We currently anticipate finalizing and recording any resulting adjustments by the end of our current fiscal year ending September 30, 2018. At the beginning of fiscal 2017, we adopted accounting guidance which requires excess tax benefits and deficiencies on share-based payments to be recorded as an income tax benefit or expense, respectively, in the consolidated statement of income rather than being recorded in additional paid-in capital on the consolidated balance sheet. As a result, we recognized income tax benefits of $1.4 million and $4.6 million in the second quarter and first half of fiscal 2018, respectively, compared to $0.9 million and $2.7 million for the prior-year periods. Also in the second quarter of fiscal 2017, the Internal Revenue Service (“IRS”) concluded their examination for fiscal years 2010 through 2013. As a result, we recognized a $1.2 million tax benefit in the second quarter of fiscal 2017, and we made payments to the IRS of approximately $20 million in the third quarter of fiscal 2017 that represented the acceleration of a deferred tax liability. In the second quarter of fiscal 2017, we also recognized a tax expense of $2.3 million to establish a reserve for an international tax position that is under examination. Excluding these discrete amounts from both periods and the one-time impacts of the TCJA, the effective tax rates for the first half of fiscal 2018 and 2017 were 28.5% and 32.5% , respectively. We evaluate the realizability of our deferred tax assets by assessing the valuation allowance and adjust the allowance, if necessary. The factors used to assess the likelihood of realization are our forecast of future taxable income and available tax planning strategies that could be implemented to realize the net deferred tax assets. The ability or failure to achieve the forecasted taxable income in the applicable taxing jurisdictions could affect the ultimate realization of deferred tax assets. Based on future operating results in certain jurisdictions, it is possible that the current valuation allowance positions of those jurisdictions could be adjusted in the next 12 months. Because of the complexity of the new Global Intangible Low-Taxed Income (“GILTI”) tax rules, we continue to evaluate this provision of the TCJA and the application of Accounting Standards Codification 740, Income Taxes. Under GAAP, we are allowed to make an accounting policy choice of either: 1) treating taxes due on future U.S. inclusions in taxable income related to GILTI as a current period expense when incurred (the “period cost method”) or 2) factoring such amounts into our measurement of our deferred taxes (the “deferred method”). Our selection of an accounting policy with respect to the new GILTI tax rules will depend, in part, on analyzing our global income to determine whether we expect to have future U.S. inclusions in taxable income related to GILTI and, if so, what the impact is expected to be. Whether we expect to have future U.S. inclusions in taxable income related to GILTI depends on not only our current structure and estimated future results of global operations, but also our intent and ability to modify our structure. We are currently in the process of analyzing our structure and, as a result, we are not yet able to reasonably estimate the effect of this provision of the TCJA. Therefore, we have not made any adjustments related to potential GILTI tax in our consolidated financial statements, and have not made a policy decision regarding whether to record deferred taxes on GILTI. As of April 1, 2018 and October 1, 2017 , the liability for income taxes associated with uncertain tax positions was $6.3 million and $6.0 million , respectively. These uncertain tax positions substantially relate to ongoing examinations, which are reasonably likely to be resolved within the next 12 months.

Reportable Segments

Reportable Segments	6 Months Ended
	Apr. 01, 2018
Segment Reporting [Abstract]
Reportable Segments	Reportable Segments Beginning in fiscal 2018 , we aligned our operations to better serve our clients and markets, resulting in two renamed reportable segments. Our GSG reportable segment primarily includes activities with U.S. government clients (federal, state and local) and all activities with development agencies worldwide. Our CIG reportable segment primarily includes activities with U.S. commercial clients and all international activities other than work for development agencies. This alignment allows us to capitalize on our growing market opportunities and enhance the development of high-end consulting and technical solutions to meet our growing client demand. We will continue to report the results of the wind-down of our non-core construction activities in the RCM segment. Prior year amounts for reportable segments have been revised to conform to the current-year presentation. GSG provides consulting and engineering services primarily to U.S. government clients (federal, state and local) and development agencies worldwide. GSG supports U.S. government civilian and defense agencies with services in water, environment, infrastructure, information technology, and emergency management services. GSG also provides engineering design services for municipal and commercial clients, especially in water infrastructure, solid waste, and high-end sustainable infrastructure designs. Additionally, GSG provides a wide range of support to development agencies worldwide. CIG provides consulting and engineering services primarily to U.S. commercial clients and international clients, both commercial and local government. CIG supports commercial clients across the Fortune 500, oil and gas, energy utilities, and mining markets. CIG also provides infrastructure and related environmental and geotechnical services, testing, engineering and project management services to commercial and local government clients across Canada, in Asia Pacific (primarily Australia and New Zealand), as well as Brazil and Chile. CIG also provides field construction management activities in the United States and Western Canada. We report the results of the wind-down of our non-core construction activities in the RCM reportable segment. The remaining backlog for RCM as of April 1, 2018 was immaterial as the related projects were substantially completed. Management evaluates the performance of these reportable segments based upon their respective segment operating income before the effect of amortization expense related to acquisitions, and other unallocated corporate expenses. We account for inter-segment revenues and transfers as if they were to third parties; that is, by applying a negotiated fee onto the costs of the services performed. All significant intercompany balances and transactions are eliminated in consolidation. Reportable Segments The following tables set forth summarized financial information regarding our reportable segments: Three Months Ended Six Months Ended April 1, April 2, April 1, April 2, (in thousands) Revenue GSG $ 406,027 $ 357,113 $ 848,799 $ 719,973 CIG 309,704 329,127 641,217 647,198 RCM 1,480 (23 ) 8,286 8,209 Elimination of inter-segment revenue (16,949 ) (22,436 ) (38,292 ) (42,748 ) Total revenue $ 700,262 $ 663,781 $ 1,460,010 $ 1,332,632 Income (loss) from operations GSG $ 34,177 $ 33,432 $ 73,302 $ 63,600 CIG 18,400 18,459 39,693 40,003 RCM (489 ) (8,466 ) (1,647 ) (11,508 ) Corporate (1) (9,372 ) (469 ) (20,043 ) (9,284 ) Total income from operations $ 42,716 $ 42,956 $ 91,305 $ 82,811 (1) Includes amortization of intangibles, other costs, and other income not allocable to our reportable segments. April 1, October 1, (in thousands) Total Assets GSG $ 488,073 $ 378,839 CIG 507,493 518,697 RCM 31,092 33,620 Corporate (1) 1,056,135 971,589 Total assets $ 2,082,793 $ 1,902,745 (1) Corporate assets consist of assets not allocated to our reportable segments including goodwill, intangible assets, deferred income taxes and certain other assets . Major Clients Other than the U.S. federal government, no single client accounted for more than 10% of our revenue. For the three and six months ended April 1, 2018 and April 2, 2017 , GSG and CIG generated revenue from all client sectors. The following table represents our revenue by client sector: Three Months Ended Six Months Ended April 1, April 2, April 1, April 2, (in thousands) Client Sector U.S. federal government (1) $ 236,951 $ 218,110 $ 473,199 $ 440,744 International (2) 178,008 197,114 342,968 369,571 U.S. state and local government 104,905 83,181 256,659 166,676 U.S. commercial 180,398 165,376 387,184 355,641 Total $ 700,262 $ 663,781 $ 1,460,010 $ 1,332,632 (1) Includes revenue generated under U.S. federal government contracts performed outside the United States. (2) Includes revenue generated from foreign operations, primarily in Canada and Australia, and revenue generated from non-U.S. clients.

Fair Value Measurements

Fair Value Measurements	6 Months Ended
	Apr. 01, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	Fair Value Measurements The fair value of long-term debt was determined using the present value of future cash flows based on the borrowing rates currently available for debt with similar terms and maturities (Level 2 measurement, as described in “Critical Accounting Policies and Estimates” in our Annual Report on Form 10-K for the fiscal year ended October 1, 2017 ). The carrying value of our long-term debt approximated fair value at April 1, 2018 and October 1, 2017 . As of April 1, 2018 , we had borrowings of $ 478.8 million outstanding under our credit agreement, which were used to fund our business acquisitions, working capital needs, stock repurchases, dividends, capital expenditures and contingent earn-outs.

Derivative Financial Instrument

Derivative Financial Instruments	6 Months Ended
	Apr. 01, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative Financial Instruments	Derivative Financial Instruments We use certain interest rate derivative contracts to hedge interest rate exposures on our variable rate debt. We enter into foreign currency derivative contracts with financial institutions to reduce the risk that cash flows and earnings will be adversely affected by foreign currency exchange rate fluctuations. Our hedging program is not designated for trading or speculative purposes. We recognize derivative instruments as either assets or liabilities on our consolidated balance sheets at fair value. We record changes in the fair value (i.e., gains or losses) of the derivatives that have been designated as cash flow hedges in our consolidated balance sheets as accumulated other comprehensive income (loss), and in our consolidated statements of income for those derivatives designated as fair value hedges. In fiscal 2013, we entered into three interest rate swap agreements that we designated as cash flow hedges to fix the variable interest rates on a portion of borrowings under our term loan facility. In the first quarter of fiscal 2014, we entered into two interest rate swap agreements that we designated as cash flow hedges to fix the variable interest rates on the borrowings under our term loan facility. At April 1, 2018 , the effective portion of our interest rate swap agreements designated as cash flow hedges before tax effect was $ (0.1) million and is expected to be reclassified from accumulated other comprehensive income (loss) to interest expense within the next 12 months. As of April 1, 2018 , the notional principal, fixed rates and related expiration dates of our outstanding interest rate swap agreements are as follows: Notional Amount (in thousands) Fixed Rate Expiration Date $38,438 1.36% May 2018 38,438 1.34% May 2018 38,438 1.35% May 2018 19,218 1.23% May 2018 19,218 1.24% May 2018 The fair values of our outstanding derivatives designated as hedging instruments were as follows: Fair Value of Derivative Instruments as of Balance Sheet Location April 1, October 1, (in thousands) Interest rate swap agreements Other current assets $ 96 $ 49 The impact of the effective portions of derivative instruments in cash flow hedging relationships and fair value relationships on income and other comprehensive income was immaterial for the first half of fiscal 2018 and the fiscal year ended October 1, 2017 . Additionally, there were no ineffective portions of derivative instruments. Accordingly, no amounts were excluded from effectiveness testing for our interest rate swap agreements.

Reclassifications Out of Accumu

Reclassifications Out of Accumulated Other Comprehensive Income (Loss)	6 Months Ended
	Apr. 01, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Reclassifications Out of Accumulated Other Comprehensive Income (Loss)	Reclassifications Out of Accumulated Other Comprehensive Income (Loss) The accumulated balances and reporting period activities for the three and six months ended April 1, 2018 and April 2, 2017 related to reclassifications out of accumulated other comprehensive loss are summarized as follows: Three Months Ended Foreign Currency Translation Adjustments Gain (Loss) on Derivative Instruments Accumulated Other Comprehensive Loss (in thousands) Balances at January 1, 2017 $ (143,026 ) $ (168 ) $ (143,194 ) Other comprehensive income before reclassifications 9,323 689 10,012 Amounts reclassified from accumulated other comprehensive income Interest rate contracts, net of tax (1) — (237 ) (237 ) Net current-period other comprehensive income 9,323 452 9,775 Balances at April 2, 2017 $ (133,703 ) $ 284 $ (133,419 ) Balances at December 31, 2017 $ (102,413 ) $ 512 $ (101,901 ) Other comprehensive loss before reclassifications (16,116 ) (136 ) (16,252 ) Amounts reclassified from accumulated other comprehensive income Interest rate contracts, net of tax (1) — 109 109 Net current-period other comprehensive loss (16,116 ) (27 ) (16,143 ) Balances at April 1, 2018 $ (118,529 ) $ 485 $ (118,044 ) Six Months Ended Foreign Currency Translation Adjustments Gain (Loss) on Derivative Instruments Accumulated Other Comprehensive Loss (in thousands) Balances at October 2, 2016 $ (126,844 ) $ (1,164 ) $ (128,008 ) Other comprehensive income (loss) before reclassifications (6,859 ) 2,027 (4,832 ) Amounts reclassified from accumulated other comprehensive income Interest rate contracts, net of tax (1) — (579 ) (579 ) Net current-period other comprehensive income (loss) (6,859 ) 1,448 (5,411 ) Balances at April 2, 2017 $ (133,703 ) $ 284 $ (133,419 ) Balances at October 1, 2017 $ (98,946 ) $ 446 $ (98,500 ) Other comprehensive loss before reclassifications (19,583 ) (52 ) (19,635 ) Amounts reclassified from accumulated other comprehensive income Interest rate contracts, net of tax (1) — 91 91 Net current-period other comprehensive income (loss) (19,583 ) 39 (19,544 ) Balances at April 1, 2018 $ (118,529 ) $ 485 $ (118,044 ) (1) This accumulated other comprehensive component is reclassified to “ Interest expense ” in our consolidated statements of income. See Note 12, “ Derivative Financial Instruments ”, for more information.

Commitments and Contingencies

Commitments and Contingencies	6 Months Ended
	Apr. 01, 2018
Commitments and Contingencies Disclosure [Abstract]
Commitments and Contingencies	Commitments and Contingencies We are subject to certain claims and lawsuits typically filed against the engineering, consulting and construction profession, alleging primarily professional errors or omissions. We carry professional liability insurance, subject to certain deductibles and policy limits, against such claims. However, in some actions, parties are seeking damages that exceed our insurance coverage or for which we are not insured. While management does not believe that the resolution of these claims will have a material adverse effect, individually or in aggregate, on our financial position, results of operations or cash flows, management acknowledges the uncertainty surrounding the ultimate resolution of these matters. The Civil Division of the United States Attorney's Office ("USAO") has informed us that it is currently evaluating claims for penalties and damages in connection with radiation remediation services provided by Tetra Tech EC, Inc., our subsidiary, at the former Navy base at Hunters Point in San Francisco. We have cooperated with the USAO and are engaged in ongoing discussions with the USAO concerning this matter. Based upon the discussions to date, we are currently unable to determine the probability of the outcome of this matter or the range of reasonably possible loss, if any.

Recent Accounting Pronouncement

Recent Accounting Pronouncements	6 Months Ended
	Apr. 01, 2018
Accounting Changes and Error Corrections [Abstract]
Recent Accounting Pronouncements	Recent Accounting Pronouncements In May 2014, the Financial Accounting Standards Board (“FASB”) issued an accounting standard that will supersede existing revenue recognition guidance under current GAAP. The new standard is a comprehensive new revenue recognition model that requires a company to recognize revenue to depict the transfer of goods or services to a customer at an amount that reflects the consideration it expects to receive in exchange for those goods and services. The standard may be applied retrospectively to each prior period presented or retrospectively with the cumulative effect recognized as of the date of initial application. This guidance is effective for fiscal reporting periods and interim periods within that reporting period, beginning after December 15, 2017 (first quarter of fiscal 2019 for us). We continue to evaluate the impact that this guidance will have on our consolidated financial statements. We expect to use the modified retrospective method, which may result in a cumulative effect adjustment as of the date of adoption. In January 2016, the FASB issued guidance that generally requires companies to measure investments in other entities, except those accounted for under the equity method, at fair value and recognize any changes in fair value in net income. The guidance is effective for fiscal years and interim periods within those fiscal years, beginning after December 15, 2017 (first quarter of fiscal 2019 for us). We do not expect the adoption of this guidance to have an impact on our consolidated financial statements. In February 2016, the FASB issued guidance that primarily requires lessees to recognize most leases on their balance sheets but record expenses on their income statements in a manner similar to current accounting. For lessors, the guidance modifies the classification criteria and the accounting for sales-type and direct financing leases. The guidance is effective for fiscal years and interim periods within those fiscal years, beginning after December 15, 2018 (first quarter of fiscal 2020 for us). Early adoption is permitted. We are currently evaluating the impact that this guidance will have on our consolidated financial statements. In June 2016, the FASB issued updated guidance which requires entities to estimate all expected credit losses for certain types of financial instruments, including trade receivables, held at the reporting date based on historical experience, current conditions, and reasonable and supportable forecasts. The updated guidance also expands the disclosure requirements to enable users of financial statements to understand the entity’s assumptions, models and methods for estimating expected credit losses. This guidance is effective for fiscal years and interim periods within those fiscal years, beginning after December 15, 2019 (first quarter of fiscal 2021 for us). Early adoption is permitted. We are currently evaluating the impact that this guidance will have on our consolidated financial statements. In August 2016, the FASB issued guidance to address eight specific cash flow issues to reduce the existing diversity in practice in how certain cash receipts and cash payments are presented and classified in the statement of cash flows. This guidance is effective for fiscal years and interim periods within those fiscal years, beginning after December 15, 2017 (first quarter of fiscal 2019 for us). Early adoption is permitted. We are currently evaluating the impact that this guidance will have on our consolidated financial statements. In October 2016, the FASB issued updated guidance which requires entities to recognize the income tax consequences of an intra-entity transfer of an asset other than inventory when the transfer occurs. This guidance is effective for fiscal reporting periods and interim reporting periods within those fiscal reporting periods, beginning after December 15, 2017 (first quarter of fiscal 2019 for us). Early adoption is permitted. We are currently evaluating the impact that this guidance will have on our consolidated financial statements. In November 2016, the FASB issued updated guidance which provides amendments to address the classification and presentation of changes in restricted cash and in the statement of cash flows. This guidance is effective for fiscal years and interim periods within those fiscal years, beginning after December 15, 2017 (first quarter of fiscal 2019 for us). Early adoption is permitted. We are currently evaluating the impact that this guidance will have on our consolidated financial statements. In January 2017, the FASB issued updated guidance to simplify the test for goodwill impairment. This guidance eliminates step two from the goodwill impairment test. Under the updated guidance, an entity should recognize an impairment charge for the amount by which the carrying amount of a reporting unit exceeds its fair value; however, the loss recognized should not exceed the total amount of goodwill allocated to the reporting unit. This guidance is effective for annual or any interim goodwill impairment tests in fiscal years beginning after December 15, 2019 (first quarter of fiscal 2021 for us), on a prospective basis. Earlier adoption is permitted for goodwill impairment tests performed on testing dates after January 1, 2017. We adopted this guidance in the first quarter of our fiscal 2018, and the adoption of this guidance had no impact on our consolidated financial statements. In May 2017, the FASB issued updated guidance to clarify when changes to the terms or conditions of a share-based payment award must be accounted for as modifications. Under the updated guidance, modification accounting is required only if the fair value, the vesting conditions, or the classification of the award changes as a result of a change in terms or conditions. This guidance is effective for fiscal years and interim periods within those fiscal years, beginning after December 15, 2017 (first quarter of fiscal 2019 for us), on a prospective basis. Early adoption is permitted. We are currently evaluating the impact that this guidance will have on our consolidated financial statements. In August 2017, the FASB issued accounting guidance on hedging activities. The amendment better aligns an entity’s risk management activities and financial reporting for hedging relationships through changes to both the designation and measurement guidance for qualifying hedging relationships and the presentation of hedge results. This guidance is effective for fiscal years and interim periods within those fiscal years, beginning after December 15, 2018 (first quarter of fiscal 2020 for us). Early adoption is permitted. We are currently evaluating the impact that this guidance will have on our consolidated financial statements. In February 2018, the FASB issued guidance on reclassification of certain tax effects from accumulated comprehensive income, which allows for a reclassification of stranded tax effects from the TCJA from accumulated other comprehensive income to retained earnings. This guidance is effective for fiscal years beginning after December 15, 2018 (first quarter of fiscal 2020 for us). We are currently evaluating the impact that this guidance will have on our consolidated financial statements.

Accounts Receivable - Net and23

Accounts Receivable - Net and Revenue Recognition (Tables)	6 Months Ended
	Apr. 01, 2018
Accounts Receivable - Net and Revenue Recognition [Abstract]
Schedule of net accounts receivable and billings in excess of costs on uncompleted contracts	Net accounts receivable and billings in excess of costs on uncompleted contracts consisted of the following: April 1, October 1, (in thousands) Billed $ 461,781 $ 376,287 Unbilled 413,517 404,899 Contract retentions 22,019 39,840 Total accounts receivable – gross 897,317 821,026 Allowance for doubtful accounts (34,098 ) (32,259 ) Total accounts receivable – net $ 863,219 $ 788,767 Billings in excess of costs on uncompleted contracts $ 144,327 $ 117,499

Goodwill and Intangible Assets

Goodwill and Intangible Assets (Tables)	6 Months Ended
	Apr. 01, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Summary of changes in the carrying value of goodwill	The following table summarizes the changes in the carrying value of goodwill: GSG CIG Total (in thousands) Balance at October 1, 2017 $ 361,761 $ 379,125 $ 740,886 Goodwill additions 26,726 54,275 81,001 Foreign exchange impact (2,138 ) (10,059 ) (12,197 ) Balance at April 1, 2018 $ 386,349 $ 423,341 $ 809,690
Summary of gross amount and accumulated amortization of acquired identifiable intangible assets with finite useful lives	The gross amount and accumulated amortization of our acquired identifiable intangible assets with finite useful lives included in “Intangible assets - net” on our consolidated balance sheets, were as follows: April 1, 2018 October 1, 2017 Weighted- Average Remaining Life (in Years) Gross Amount Accumulated Amortization Gross Amount Accumulated Amortization ($ in thousands) Non-compete agreements 0.0 $ 83 $ (83 ) $ 495 $ (493 ) Client relations 2.7 54,643 (43,119 ) 90,297 (75,074 ) Backlog 0.8 23,184 (16,443 ) 21,518 (13,301 ) Trade names 2.8 5,248 (2,666 ) 6,685 (3,439 ) Total $ 83,158 $ (62,311 ) $ 118,995 $ (92,307 )
Schedule of estimated amortization expense for remainder of fiscal and succeeding years	Estimated amortization expense for the remainder of fiscal 2018 and succeeding years is as follows: Amount (in thousands) 2018 $ 8,300 2019 7,471 2020 2,786 2021 1,650 2022 411 Beyond 229 Total $ 20,847

Property and Equipment (Tables)

Property and Equipment (Tables)	6 Months Ended
	Apr. 01, 2018
Property, Plant and Equipment [Abstract]
Schedule of property and equipment	Property and equipment consisted of the following: April 1, October 1, (in thousands) Equipment, furniture and fixtures 147,316 150,026 Leasehold improvements 31,066 27,689 Land and buildings $ 3,746 $ 3,680 Total property and equipment 182,128 181,395 Accumulated depreciation (126,633 ) (124,560 ) Property and equipment, net $ 55,495 $ 56,835

Earnings per Share ("EPS") (Tab

Earnings per Share ("EPS") (Tables)	6 Months Ended
	Apr. 01, 2018
Earnings Per Share [Abstract]
Schedule of number of weighted-average shares used to compute basic and diluted EPS	The following table sets forth the number of weighted-average shares used to compute basic and diluted EPS: Three Months Ended Six Months Ended April 1, April 2, April 1, April 2, (in thousands, except per share data) Net income attributable to Tetra Tech $ 28,725 $ 26,862 $ 74,759 $ 53,424 Weighted-average common shares outstanding – basic 55,841 57,270 55,900 57,171 Effect of dilutive stock options and unvested restricted stock 832 1,000 925 1,023 Weighted-average common stock outstanding – diluted 56,673 58,270 56,825 58,194 Earnings per share attributable to Tetra Tech: Basic $ 0.51 $ 0.47 $ 1.34 $ 0.93 Diluted $ 0.51 $ 0.46 $ 1.32 $ 0.92

Reportable Segments (Tables)

Reportable Segments (Tables)	6 Months Ended
	Apr. 01, 2018
Segment Reporting [Abstract]
Summarized financial information of reportable segments	The following tables set forth summarized financial information regarding our reportable segments: Three Months Ended Six Months Ended April 1, April 2, April 1, April 2, (in thousands) Revenue GSG $ 406,027 $ 357,113 $ 848,799 $ 719,973 CIG 309,704 329,127 641,217 647,198 RCM 1,480 (23 ) 8,286 8,209 Elimination of inter-segment revenue (16,949 ) (22,436 ) (38,292 ) (42,748 ) Total revenue $ 700,262 $ 663,781 $ 1,460,010 $ 1,332,632 Income (loss) from operations GSG $ 34,177 $ 33,432 $ 73,302 $ 63,600 CIG 18,400 18,459 39,693 40,003 RCM (489 ) (8,466 ) (1,647 ) (11,508 ) Corporate (1) (9,372 ) (469 ) (20,043 ) (9,284 ) Total income from operations $ 42,716 $ 42,956 $ 91,305 $ 82,811 (1) Includes amortization of intangibles, other costs, and other income not allocable to our reportable segments. April 1, October 1, (in thousands) Total Assets GSG $ 488,073 $ 378,839 CIG 507,493 518,697 RCM 31,092 33,620 Corporate (1) 1,056,135 971,589 Total assets $ 2,082,793 $ 1,902,745 (1) Corporate assets consist of assets not allocated to our reportable segments including goodwill, intangible assets, deferred income taxes and certain other assets .
Summary of revenue by client sector	The following table represents our revenue by client sector: Three Months Ended Six Months Ended April 1, April 2, April 1, April 2, (in thousands) Client Sector U.S. federal government (1) $ 236,951 $ 218,110 $ 473,199 $ 440,744 International (2) 178,008 197,114 342,968 369,571 U.S. state and local government 104,905 83,181 256,659 166,676 U.S. commercial 180,398 165,376 387,184 355,641 Total $ 700,262 $ 663,781 $ 1,460,010 $ 1,332,632 (1) Includes revenue generated under U.S. federal government contracts performed outside the United States. (2) Includes revenue generated from foreign operations, primarily in Canada and Australia, and revenue generated from non-U.S. clients.

Derivative Financial Instrume28

Derivative Financial Instruments (Tables)	6 Months Ended
	Apr. 01, 2018
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Schedule of notional principal, fixed rates and related expiration dates of outstanding interest rate swap agreements	As of April 1, 2018 , the notional principal, fixed rates and related expiration dates of our outstanding interest rate swap agreements are as follows: Notional Amount (in thousands) Fixed Rate Expiration Date $38,438 1.36% May 2018 38,438 1.34% May 2018 38,438 1.35% May 2018 19,218 1.23% May 2018 19,218 1.24% May 2018
Schedule of fair values of the entity's outstanding derivatives designated as hedging instruments	The fair values of our outstanding derivatives designated as hedging instruments were as follows: Fair Value of Derivative Instruments as of Balance Sheet Location April 1, October 1, (in thousands) Interest rate swap agreements Other current assets $ 96 $ 49

Reclassifications Out of Accu29

Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Tables)	6 Months Ended
	Apr. 01, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Summary of reclassifications out of accumulated other comprehensive loss	The accumulated balances and reporting period activities for the three and six months ended April 1, 2018 and April 2, 2017 related to reclassifications out of accumulated other comprehensive loss are summarized as follows: Three Months Ended Foreign Currency Translation Adjustments Gain (Loss) on Derivative Instruments Accumulated Other Comprehensive Loss (in thousands) Balances at January 1, 2017 $ (143,026 ) $ (168 ) $ (143,194 ) Other comprehensive income before reclassifications 9,323 689 10,012 Amounts reclassified from accumulated other comprehensive income Interest rate contracts, net of tax (1) — (237 ) (237 ) Net current-period other comprehensive income 9,323 452 9,775 Balances at April 2, 2017 $ (133,703 ) $ 284 $ (133,419 ) Balances at December 31, 2017 $ (102,413 ) $ 512 $ (101,901 ) Other comprehensive loss before reclassifications (16,116 ) (136 ) (16,252 ) Amounts reclassified from accumulated other comprehensive income Interest rate contracts, net of tax (1) — 109 109 Net current-period other comprehensive loss (16,116 ) (27 ) (16,143 ) Balances at April 1, 2018 $ (118,529 ) $ 485 $ (118,044 ) Six Months Ended Foreign Currency Translation Adjustments Gain (Loss) on Derivative Instruments Accumulated Other Comprehensive Loss (in thousands) Balances at October 2, 2016 $ (126,844 ) $ (1,164 ) $ (128,008 ) Other comprehensive income (loss) before reclassifications (6,859 ) 2,027 (4,832 ) Amounts reclassified from accumulated other comprehensive income Interest rate contracts, net of tax (1) — (579 ) (579 ) Net current-period other comprehensive income (loss) (6,859 ) 1,448 (5,411 ) Balances at April 2, 2017 $ (133,703 ) $ 284 $ (133,419 ) Balances at October 1, 2017 $ (98,946 ) $ 446 $ (98,500 ) Other comprehensive loss before reclassifications (19,583 ) (52 ) (19,635 ) Amounts reclassified from accumulated other comprehensive income Interest rate contracts, net of tax (1) — 91 91 Net current-period other comprehensive income (loss) (19,583 ) 39 (19,544 ) Balances at April 1, 2018 $ (118,529 ) $ 485 $ (118,044 ) (1) This accumulated other comprehensive component is reclassified to “ Interest expense ” in our consolidated statements of income. See Note 12, “ Derivative Financial Instruments ”, for more information.

Basis of Presentation (Details)

Basis of Presentation (Details)	6 Months Ended
	Apr. 01, 2018segment
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Number of renamed reportable segments	2

Accounts Receivable - Net and31

Accounts Receivable - Net and Revenue Recognition - Net Accounts Receivable and Billings in Excess of Costs on Uncompleted Contracts (Details) - USD ($) $ in Thousands	Apr. 01, 2018	Oct. 01, 2017
Accounts Receivable - Net and Revenue Recognition [Abstract]
Billed	$ 461,781	$ 376,287
Unbilled	413,517	404,899
Contract retentions	22,019	39,840
Total accounts receivable – gross	897,317	821,026
Allowance for doubtful accounts	(34,098)	(32,259)
Total accounts receivable – net	863,219	788,767
Billings in excess of costs on uncompleted contracts	$ 144,327	$ 117,499

Accounts Receivable - Net and32

Accounts Receivable - Net and Revenue Recognition (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Apr. 01, 2018	Apr. 02, 2017	Apr. 01, 2018	Apr. 02, 2017	Oct. 01, 2017
Accounts Receivable - Net and Revenue Recognition
Period for billing and collecting unbilled receivables			12 months
Period for earning majority of billings in excess of costs			12 months
Unbilled accounts receivable related to claims and requests for equitable adjustment on contracts	$ 65		$ 65		$ 59
Gains (losses) from claim settlement			0
Billed accounts receivable related to U.S. federal government contracts	89.3		89.3		45.4
U.S. federal government unbilled receivables	104.5		$ 104.5		$ 109.7
Threshold percentage for disclosure of accounts receivable from a single client			10.00%		10.00%
Net unfavorable operating income adjustments				$ 8
Liabilities for anticipated losses	4.4		$ 4.4		$ 8.1
Estimated cost to complete the related contracts	4.6		4.6
CIG
Accounts Receivable - Net and Revenue Recognition
Net unfavorable operating income adjustments	$ 0.7		$ 1.4	2.3
RCM
Accounts Receivable - Net and Revenue Recognition
Gains (losses) from claim settlement				(3.2)
Reduction in the revenue related to the evaluation of the claims				4.5
Net unfavorable operating income adjustments		$ 4		$ 5.7

Mergers and Acquisitions - Narr

Mergers and Acquisitions - Narrative (Details)	3 Months Ended				6 Months Ended		12 Months Ended
	Apr. 01, 2018USD ($)employee	Dec. 31, 2017USD ($)employee	Apr. 02, 2017USD ($)employee	Mar. 27, 2016USD ($)	Apr. 01, 2018USD ($)	Apr. 02, 2017USD ($)	Oct. 02, 2016USD ($)
Mergers and Acquisitions
Contingent earn-out liability	$ 30,100,000				$ 30,100,000
Aggregate maximum of contingent consideration	$ 43,400,000				43,400,000
Increase (decrease) in contingent consideration					1,900,000	$ (7,100,000)
Gain (loss) on change in contingent consideration					$ (1,900,000)	7,100,000
Minimum
Mergers and Acquisitions
Earn-out period					2 years
Minimum | Existing customer contracts
Mergers and Acquisitions
Useful life of intangible assets					1 year
Minimum | Trade names
Mergers and Acquisitions
Useful life of intangible assets					3 years
Maximum
Mergers and Acquisitions
Earn-out period					3 years
Maximum | Existing customer contracts
Mergers and Acquisitions
Useful life of intangible assets					10 years
Maximum | Trade names
Mergers and Acquisitions
Useful life of intangible assets					5 years
Norman Disney & Young
Mergers and Acquisitions
Number of employees acquired in acquisition | employee	700
Fair value of acquisition purchase price	$ 56,100,000
Cash paid to the former owners	46,300,000
Amount held in escrow	1,600,000				$ 1,600,000
Purchase price payable	600,000
Contingent earn-out liability	7,600,000				7,600,000
Aggregate maximum of contingent consideration	$ 20,200,000				$ 20,200,000
Earn-out period	3 years
Glumac
Mergers and Acquisitions
Number of employees acquired in acquisition | employee		300
Fair value of acquisition purchase price		$ 38,400,000
Cash paid to the former owners		19,000,000
Amount held in escrow		1,000,000
Contingent earn-out liability		18,400,000
Aggregate maximum of contingent consideration		$ 20,000,000
Earn-out period		3 years
Eco Logical Australia
Mergers and Acquisitions
Number of employees acquired in acquisition | employee			160
Fair value of acquisition purchase price			$ 9,900,000
Cash paid to the former owners			8,300,000
Contingent earn-out liability			1,600,000			1,600,000
Aggregate maximum of contingent consideration			$ 1,700,000			1,700,000
Earn-out period			2 years
INDUS
Mergers and Acquisitions
Contingent earn-out liability			$ 0	$ 4,700,000		0	$ 4,900,000
Aggregate maximum of contingent consideration				$ 8,000,000
Earn-out period				2 years
Increase (decrease) in contingent consideration						(5,000,000)
Gain (loss) on change in contingent consideration			5,000,000			5,000,000
Potential earn-out to be paid each year				$ 4,000,000
Estimated potential earn-out (as a percent)				50.00%			59.00%
Minimum operating income threshold to earn contingent consideration during year one			3,200,000	$ 3,200,000
Minimum operating income threshold to earn contingent consideration during year two			$ 3,600,000	3,600,000
Operating income to earn maximum consideration during the first year				3,600,000
Operating income to earn maximum consideration during the second year				$ 4,000,000
CEG
Mergers and Acquisitions
Increase (decrease) in contingent consideration						(2,100,000)
Gain (loss) on change in contingent consideration						$ 2,100,000

Goodwill and Intangible Asset34

Goodwill and Intangible Assets - Goodwill (Details) $ in Thousands	6 Months Ended
	Apr. 01, 2018USD ($)
Goodwill
Balance at beginning of the period	$ 740,886
Goodwill additions	81,001
Foreign exchange impact	(12,197)
Balance at end of the period	809,690
GSG
Goodwill
Balance at beginning of the period	361,761
Goodwill additions	26,726
Foreign exchange impact	(2,138)
Balance at end of the period	386,349
CIG
Goodwill
Balance at beginning of the period	379,125
Goodwill additions	54,275
Foreign exchange impact	(10,059)
Balance at end of the period	$ 423,341

Goodwill and Intangible Asset35

Goodwill and Intangible Assets - Narrative (Details) - USD ($)	Jul. 03, 2017	Apr. 01, 2018	Oct. 01, 2017
Goodwill [Line Items]
Impairment of goodwill	$ 0
GSG
Goodwill [Line Items]
Gross amounts of goodwill		$ 404,000,000	$ 379,500,000
Accumulated impairment		17,700,000	17,700,000
CIG
Goodwill [Line Items]
Gross amounts of goodwill		521,300,000	477,000,000
Accumulated impairment		$ 97,900,000	$ 97,900,000
Income approach
Goodwill [Line Items]
Weighted rate used in fair value of goodwill (as a percent)		70.00%
Discounted cash flow and market approach
Goodwill [Line Items]
Weighted rate used in fair value of goodwill (as a percent)		30.00%

Goodwill and Intangible Asset36

Goodwill and Intangible Assets - Finite Lived Assets (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Apr. 01, 2018	Apr. 02, 2017	Apr. 01, 2018	Apr. 02, 2017	Oct. 01, 2017
Finite-lived intangible assets
Gross Amount	$ 83,158		$ 83,158		$ 118,995
Accumulated Amortization	(62,311)		(62,311)		(92,307)
Amortization expense	5,000	$ 5,900	9,700	$ 11,800
Estimated amortization expense
2,018	8,300		8,300
2,019	7,471		7,471
2,020	2,786		2,786
2,021	1,650		1,650
2,022	411		411
Beyond	229		229
Total	20,847		$ 20,847
Non-compete agreements
Finite-lived intangible assets
Weighted- Average Remaining Life (in Years)			0 years
Gross Amount	83		$ 83		495
Accumulated Amortization	(83)		$ (83)		(493)
Client relations
Finite-lived intangible assets
Weighted- Average Remaining Life (in Years)			2 years 8 months 12 days
Gross Amount	54,643		$ 54,643		90,297
Accumulated Amortization	(43,119)		$ (43,119)		(75,074)
Backlog
Finite-lived intangible assets
Weighted- Average Remaining Life (in Years)			9 months 18 days
Gross Amount	23,184		$ 23,184		21,518
Accumulated Amortization	(16,443)		$ (16,443)		(13,301)
Trade names
Finite-lived intangible assets
Weighted- Average Remaining Life (in Years)			2 years 9 months 18 days
Gross Amount	5,248		$ 5,248		6,685
Accumulated Amortization	$ (2,666)		$ (2,666)		$ (3,439)

Property and Equipment (Details

Property and Equipment (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Apr. 01, 2018	Apr. 02, 2017	Apr. 01, 2018	Apr. 02, 2017	Oct. 01, 2017
Property and Equipment
Property and equipment, gross	$ 182,128		$ 182,128		$ 181,395
Accumulated depreciation	(126,633)		(126,633)		(124,560)
Property and equipment, net	55,495		55,495		56,835
Depreciation expense related to property and equipment	5,100	$ 5,800	10,300	$ 11,000
Equipment, furniture and fixtures
Property and Equipment
Property and equipment, gross	147,316		147,316		150,026
Leasehold improvements
Property and Equipment
Property and equipment, gross	31,066		31,066		27,689
Land and buildings
Property and Equipment
Property and equipment, gross	$ 3,746		$ 3,746		$ 3,680

Stock Repurchase and Dividends

Stock Repurchase and Dividends (Details) - USD ($)	Apr. 30, 2018	Jan. 29, 2018	Nov. 07, 2017	Apr. 01, 2018	Apr. 02, 2017	Nov. 07, 2016
Subsequent Event [Line Items]
Maximum repurchase amount under stock repurchase program						$ 200,000,000
Shares repurchased through open market purchases (in shares)				1,020,033
Average price of shares repurchased (in dollars per share)				$ 49.02
Cost of shares repurchased				$ 50,000,000
Quarterly cash dividend declared (in dollars per share)		$ 0.10	$ 0.10
Dividends paid				$ 11,172,000	$ 10,301,000
Subsequent Event
Subsequent Event [Line Items]
Quarterly cash dividend declared (in dollars per share)	$ 0.12

Stockholders' Equity and Stoc39

Stockholders' Equity and Stock Compensation Plans (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Apr. 01, 2018	Apr. 02, 2017	Apr. 01, 2018	Apr. 02, 2017
Stockholders' Equity and Stock Compensation Plans
Stock-based compensation expense	$ 4.7	$ 3.4	$ 8.7	$ 6.6
Non-employee directors and executive officers
Stockholders' Equity and Stock Compensation Plans
Options granted (in shares)			170,222
Exercise price of stock options granted (in dollars per share)			$ 47.95
Weighted-average fair value of stock options granted (in dollars per share)			$ 14.79
Stock options | Executive officers
Stockholders' Equity and Stock Compensation Plans
Vesting period			4 years
Stock options | Non-employee director
Stockholders' Equity and Stock Compensation Plans
Vesting period			1 year
Performance-based restricted stock
Stockholders' Equity and Stock Compensation Plans
Percentage of shares that ultimately vest depending on growth in diluted earnings per share			50.00%
Percentage of shares that ultimately vest depending on the shareholder return relative to peer group of companies over vesting period			50.00%
Performance-based restricted stock | Non-employee directors and executive officers
Stockholders' Equity and Stock Compensation Plans
Vesting period			3 years
Granted (in shares)			98,599
Granted, fair value (in dollars per share)			$ 56.10
Restricted stock units | Non-employee directors, executive officers and employees
Stockholders' Equity and Stock Compensation Plans
Granted (in shares)			189,651
Granted, fair value (in dollars per share)			$ 48.01
Restricted stock units | Non-employee director
Stockholders' Equity and Stock Compensation Plans
Vesting period			1 year
Restricted stock units | Executive officers and employees
Stockholders' Equity and Stock Compensation Plans
Vesting period			4 years

Earnings per Share ("EPS") - Ca

Earnings per Share ("EPS") - Calculation (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Apr. 01, 2018	Apr. 02, 2017	Apr. 01, 2018	Apr. 02, 2017
Earnings Per Share [Abstract]
Net income attributable to Tetra Tech	$ 28,725	$ 26,862	$ 74,759	$ 53,424
Weighted-average common shares outstanding – basic (in shares)	55,841	57,270	55,900	57,171
Effect of dilutive stock options and unvested restricted stock (in shares)	832	1,000	925	1,023
Weighted-average common stock outstanding – diluted (in shares)	56,673	58,270	56,825	58,194
Earnings per share attributable to Tetra Tech:
Basic (in dollars per share)	$ 0.51	$ 0.47	$ 1.34	$ 0.93
Diluted (in dollars per share)	$ 0.51	$ 0.46	$ 1.32	$ 0.92

Earnings per Share ("EPS") - An

Earnings per Share ("EPS") - Antidilutive Securities (Details) - shares	3 Months Ended		6 Months Ended
	Apr. 01, 2018	Apr. 02, 2017	Apr. 01, 2018	Apr. 02, 2017
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Securities excluded from the calculation of dilutive potential common shares (in shares)	0	0	0	0

Income Taxes (Details)

Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended		12 Months Ended
	Apr. 01, 2018	Dec. 31, 2017	Jul. 02, 2017	Apr. 02, 2017	Apr. 01, 2018	Apr. 02, 2017	Sep. 30, 2018	Oct. 01, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Effective tax rate (as a percent)					11.00%	30.40%
Deferred tax benefit from tax reform remeasurement		$ 10.1
Effective income tax rate excluding revaluation of deferred taxes (as a percent)					23.00%
Tax benefit recognized in connection with the IRS examination				$ 1.2
Payments made to tax authorities			$ 20
Effective income tax rate excluding excess tax benefit and TCJA impacts (as a percent)					28.50%	32.50%
Liability for uncertain tax positions	$ 6.3				$ 6.3			$ 6
Period during which unrecognized tax benefits would affect the effective tax rate					12 months
Accounting Standards Update 2016-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Excess tax benefit	$ 1.4			0.9	$ 4.6	$ 2.7
Scenario, Forecast
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Statutory federal tax rate							24.50%
Foreign Tax Authority
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reserve for uncertain tax position in connection with IRS examination				$ 2.3

Reportable Segments - Financial

Reportable Segments - Financial Information (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Apr. 01, 2018USD ($)	Apr. 02, 2017USD ($)	Apr. 01, 2018USD ($)segment	Apr. 02, 2017USD ($)	Oct. 01, 2017USD ($)
Financial information regarding reportable segments
Number of renamed reportable segments | segment			2
Revenue	$ 700,262	$ 663,781	$ 1,460,010	$ 1,332,632
Income (loss) from operations	42,716	42,956	91,305	82,811
Total Assets	2,082,793		2,082,793		$ 1,902,745
Operating segment | GSG
Financial information regarding reportable segments
Revenue	406,027	357,113	848,799	719,973
Income (loss) from operations	34,177	33,432	73,302	63,600
Total Assets	488,073		488,073		378,839
Operating segment | CIG
Financial information regarding reportable segments
Revenue	309,704	329,127	641,217	647,198
Income (loss) from operations	18,400	18,459	39,693	40,003
Total Assets	507,493		507,493		518,697
Operating segment | RCM
Financial information regarding reportable segments
Revenue	1,480	(23)	8,286	8,209
Income (loss) from operations	(489)	(8,466)	(1,647)	(11,508)
Total Assets	31,092		31,092		33,620
Elimination of inter-segment revenue
Financial information regarding reportable segments
Revenue	(16,949)	(22,436)	(38,292)	(42,748)
Corporate
Financial information regarding reportable segments
Income (loss) from operations	(9,372)	$ (469)	(20,043)	$ (9,284)
Total Assets	$ 1,056,135		$ 1,056,135		$ 971,589

Reportable Segments - Revenue B

Reportable Segments - Revenue By Client Sector (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Apr. 01, 2018	Apr. 02, 2017	Apr. 01, 2018	Apr. 02, 2017
Revenue by client sector
Threshold percentage for disclosure of revenue from a single client	10.00%	10.00%	10.00%	10.00%
Revenue	$ 700,262	$ 663,781	$ 1,460,010	$ 1,332,632
U.S. federal government
Revenue by client sector
Revenue	236,951	218,110	473,199	440,744
International
Revenue by client sector
Revenue	178,008	197,114	342,968	369,571
U.S. state and local government
Revenue by client sector
Revenue	104,905	83,181	256,659	166,676
U.S. commercial
Revenue by client sector
Revenue	$ 180,398	$ 165,376	$ 387,184	$ 355,641

Fair Value Measurements (Detail

Fair Value Measurements (Details) $ in Millions	Apr. 01, 2018USD ($)
Fair Value Disclosures [Abstract]
Borrowing under credit agreement	$ 478.8

Derivative Financial Instrume46

Derivative Financial Instruments - General Information (Details)	3 Months Ended	6 Months Ended	12 Months Ended
	Dec. 29, 2013agreement	Apr. 01, 2018USD ($)	Oct. 01, 2017USD ($)	Sep. 29, 2013agreement
Interest rate swap agreements | Designated as cash flow hedges | Derivatives designated as hedging instruments
Derivative Financial Instruments
Number of derivative agreements | agreement	2			3
Amount of effective portion of derivatives before tax effect		$ (100,000)
Period of reclassification from accumulated other comprehensive income (loss) to interest expense		12 months
Interest rate swap agreement bearing fixed rate 1.36%
Derivative Financial Instruments
Notional Amount		$ 38,438,000
Fixed Rate (as a percent)		1.36%
Interest rate swap agreement bearing fixed rate 1.34%
Derivative Financial Instruments
Notional Amount		$ 38,438,000
Fixed Rate (as a percent)		1.34%
Interest rate swap agreement bearing fixed rate 1.35%
Derivative Financial Instruments
Notional Amount		$ 38,438,000
Fixed Rate (as a percent)		1.35%
Interest rate swap agreement bearing fixed rate 1.23%
Derivative Financial Instruments
Notional Amount		$ 19,218,000
Fixed Rate (as a percent)		1.23%
Interest rate swap agreement bearing fixed rate 1.24%
Derivative Financial Instruments
Notional Amount		$ 19,218,000
Fixed Rate (as a percent)		1.24%
Foreign currency forward contracts and interest rate swap agreements | Derivatives designated as hedging instruments
Derivative Financial Instruments
Ineffective portion of derivative instruments		$ 0	$ 0
Amounts excluded from effectiveness testing		$ 0	$ 0

Derivative Financial Instrume47

Derivative Financial Instruments - Fair Value (Details) - USD ($) $ in Thousands	Apr. 01, 2018	Oct. 01, 2017
Interest rate swap agreements | Derivatives designated as hedging instruments | Designated as cash flow hedges | Other current assets
Derivative Financial Instruments
Fair Value of Derivative Instruments	$ 96	$ 49

Reclassifications Out of Accu48

Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Apr. 01, 2018	Apr. 02, 2017	Apr. 01, 2018	Apr. 02, 2017
Reclassifications out of accumulated other comprehensive loss
Balance at the beginning of the period			$ 928,453
Other comprehensive income (loss) before reclassifications	$ (16,252)	$ 10,012	(19,635)	$ (4,832)
Amounts reclassified from accumulated other comprehensive income	109	(237)	91	(579)
Net current-period other comprehensive income (loss)	(16,143)	9,775	(19,544)	(5,411)
Balance at the end of the period	940,046		940,046
Foreign Currency Translation Adjustments
Reclassifications out of accumulated other comprehensive loss
Balance at the beginning of the period	(102,413)	(143,026)	(98,946)	(126,844)
Other comprehensive income (loss) before reclassifications	(16,116)	9,323	(19,583)	(6,859)
Amounts reclassified from accumulated other comprehensive income	0	0	0	0
Net current-period other comprehensive income (loss)	(16,116)	9,323	(19,583)	(6,859)
Balance at the end of the period	(118,529)	(133,703)	(118,529)	(133,703)
Gain (Loss) on Derivative Instruments
Reclassifications out of accumulated other comprehensive loss
Balance at the beginning of the period	512	(168)	446	(1,164)
Other comprehensive income (loss) before reclassifications	(136)	689	(52)	2,027
Amounts reclassified from accumulated other comprehensive income	109	(237)	91	(579)
Net current-period other comprehensive income (loss)	(27)	452	39	1,448
Balance at the end of the period	485	284	485	284
Accumulated Other Comprehensive Loss
Reclassifications out of accumulated other comprehensive loss
Balance at the beginning of the period	(101,901)	(143,194)	(98,500)	(128,008)
Balance at the end of the period	$ (118,044)	$ (133,419)	$ (118,044)	$ (133,419)