Debt Instrument, Annual Principal Payment, Amortization Percentage

No definition available.

Name:	ttek_DebtInstrumentAnnualPrincipalPaymentAmortizationPercentage
Namespace Prefix:	ttek_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line Of Credit Facility, Accordion Feature, Higher Borrowing Capacity Option

+ References

No definition available.

+ Details

Name:	ttek_LineOfCreditFacilityAccordionFeatureHigherBorrowingCapacityOption
Namespace Prefix:	ttek_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line Of Credit Facility, Accordion Feature, Increase Limit

+ References

No definition available.

+ Details

Name:	ttek_LineOfCreditFacilityAccordionFeatureIncreaseLimit
Namespace Prefix:	ttek_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line Of Credit Facility, Covenant, Consolidated Interest Coverage Ratio, Minimum

+ References

No definition available.

+ Details

Name:	ttek_LineOfCreditFacilityCovenantConsolidatedInterestCoverageRatioMinimum
Namespace Prefix:	ttek_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Line Of Credit Facility, Covenant, Consolidated Leverage Ratio, Maximum

+ References

No definition available.

+ Details

Name:	ttek_LineOfCreditFacilityCovenantConsolidatedLeverageRatioMaximum
Namespace Prefix:	ttek_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Weighted average interest rate of long-term debt outstanding including the effects of interest rate swap agreement.

+ References

No definition available.

+ Details

Name:	ttek_LongTermDebtWeightedAverageInterestRateIncludingEffectsOfInterestRateSwap
Namespace Prefix:	ttek_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the consolidated fixed charge coverage ratio as defined by the loan agreement.

+ References

No definition available.

+ Details

Name:	ttek_RatioOfConsolidatedFixedChargeCoverageRatio
Namespace Prefix:	ttek_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

Name:	us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

Name:	us-gaap_LineOfCredit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_LongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Weighted average interest rate of long-term debt outstanding calculated at point in time.

+ References

No definition available.

+ Details

Name:	us-gaap_LongtermDebtWeightedAverageInterestRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Indebtedness divided by net capital.

+ References

No definition available.

+ Details

Name:	us-gaap_RatioOfIndebtednessToNetCapital1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_StandbyLettersOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=ttek_LineOfCreditMulticurrencyBorrowingsAndLettersOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_SecuredDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ttek_AmendedCreditAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_FederalFundsEffectiveSwapRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=ttek_TermLoanFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_EurodollarMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=ttek_PrimeRateOrEurodollarRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_BridgeLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=ttek_CreditAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Long-Term Debt - Narrative (Details)	Jul. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2018 AUD ($)
Long-term debt
Weighted-average rate including the effects of interest rate swap agreement (as a percent)		3.28%	3.28%
Amount outstanding under credit facility		$ 277,100,000
Standby Letters of Credit
Long-term debt
Maximum borrowing capacity			$ 30,000,000
Letters of credit outstanding		29,800,000
Amount outstanding under credit facility		0
Multicurrency borrowings and letter of credit
Long-term debt
Letters of credit outstanding		4,300,000
Bank guarantees
Long-term debt
Amount outstanding under credit facility		7,100,000
Amended Credit Agreement
Long-term debt
Accordion feature, higher borrowing capacity option	$ 1,000,000,000
Maximum borrowing capacity	$ 700,000,000
Debt instrument term	5 years
Accordion feature, increase limit	$ 300,000,000
Annual principal payment, amortization percentage	5.00%
Borrowings outstanding		$ 277,100,000
Weighted-average interest rate (as a percent)		3.27%	3.27%
Debt covenant, maximum consolidated leverage ratio	3.00
Debt covenant, minimum consolidated interest coverage ratio	3.00
Amended Credit Agreement \| Federal Funds Effective Swap Rate
Long-term debt
Basis spread on variable rate	0.50%
Amended Credit Agreement \| Term Loan Facility
Long-term debt
Maximum borrowing capacity	$ 250,000,000
Borrowings outstanding		$ 250,000,000
Amended Credit Agreement \| Revolving Credit Facility
Long-term debt
Maximum borrowing capacity	$ 450,000,000
Borrowings outstanding		27,100,000
Amount available for borrowing under facility		422,000,000
Amended Credit Agreement \| Revolving Credit Facility \| Eurodollar \| Minimum
Long-term debt
Basis spread on variable rate	1.00%
Amended Credit Agreement \| Revolving Credit Facility \| Eurodollar \| Maximum
Long-term debt
Basis spread on variable rate	1.75%
Amended Credit Agreement \| Revolving Credit Facility \| Prime Rate or Eurodollar Rate
Long-term debt
Basis spread on variable rate	1.00%
Amended Credit Agreement \| Revolving Credit Facility \| Prime Rate or Eurodollar Rate \| Minimum
Long-term debt
Basis spread on variable rate	0.00%
Amended Credit Agreement \| Revolving Credit Facility \| Prime Rate or Eurodollar Rate \| Maximum
Long-term debt
Basis spread on variable rate	0.75%
Amended Credit Agreement \| Standby Letters of Credit
Long-term debt
Maximum borrowing capacity	$ 100,000,000
Letters of credit outstanding		$ 900,000
Amended Credit Agreement \| Bridge Loan
Long-term debt
Maximum borrowing capacity	20,000,000
Amended Credit Agreement \| Multicurrency borrowings and letter of credit
Long-term debt
Maximum borrowing capacity	$ 200,000,000
Credit Agreement
Long-term debt
Consolidated leverage ratio		1.23	1.23
Consolidated fixed charge coverage ratio		15.42	15.42