Significant Accounting Policies - Asset Retirement Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
Sep. 29, 2018	Sep. 30, 2017
Property, Plant and Equipment [Line Items]
Asset retirement obligation gross expected future cash flows	$ 6,600
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset retirement liability, beginning of period	5,382	$ 2,796
Payment of asset retirement obligations		(175)
Adjustment to asset retirement obligations recognized		213
Adjustment to asset retirement obligations recognized	(123)
Additional asset retirement obligations due to acquisition	466	2,325
Accretion recognized	156	151
Changes due to foreign currency exchange	(79)	72
Asset retirement liability, end of period	5,802	$ 5,382
Other current liabilities:
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset retirement liability, end of period	1,273
Other long-term liabilities
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]
Asset retirement liability, end of period	$ 4,529

Additional asset retirement obligation related to acquisition

No definition available.

Name:	cohr_Additionalassetretirementobligationrelatedtoacquisition
Namespace Prefix:	cohr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Asset Retirement Obligation, Adjustments To Obligations Recognized

+ References

No definition available.

+ Details

Name:	cohr_AssetRetirementObligationAdjustmentsToObligationsRecognized
Namespace Prefix:	cohr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Asset retirement obligation gross expected future cash flows

+ References

No definition available.

+ Details

Name:	cohr_Assetretirementobligationgrossexpectedfuturecashflows
Namespace Prefix:	cohr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

+ References

+ Details

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

- Definition

Amount of accretion expense recognized during the period that is associated with an asset retirement obligation. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6392676&loc=d3e7480-110848

+ Details

Name:	us-gaap_AssetRetirementObligationAccretionExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of foreign currency translation gain (loss) which decreases (increases) asset retirement obligations.

+ References

+ Details

Name:	us-gaap_AssetRetirementObligationForeignCurrencyTranslationGainLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of asset retirement obligations incurred during the period.

+ References

+ Details

Name:	us-gaap_AssetRetirementObligationLiabilitiesIncurred
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset.

+ References

+ Details

Name:	us-gaap_AssetRetirementObligationLiabilitiesSettled
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_OtherCurrentLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_OtherNoncurrentLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: