Debt (Details) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($)
Term Loan and Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Maximum debt to equity ratio under covenants	0.40
Term Loan and Revolving Credit Facility [Member] \| Base Rate [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	0.00%
Term Loan and Revolving Credit Facility [Member] \| Minimum [Member] \| Eurocurrency [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	0.75%
Term Loan and Revolving Credit Facility [Member] \| Minimum [Member] \| Eurocurrency Without Debt Ratings [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	0.875%
Term Loan and Revolving Credit Facility [Member] \| Maximum [Member] \| Eurocurrency [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.125%
Term Loan and Revolving Credit Facility [Member] \| Maximum [Member] \| Eurocurrency Without Debt Ratings [Member]
Debt Instrument [Line Items]
Basis spread on variable rate	1.00%
Term Loan [Member]
Debt Instrument [Line Items]
Principal amount of debt	$ 1,000
Interest rate at period end	2.40%
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit, maximum borrowing capacity	$ 750
Line of Credit Facility, Maximum Period Drawn Notes Have Been Outstanding	90 days

Debt Instrument, Restrictive Covenants, Maximum Debt to Equity Ratio

No definition available.

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Namespace Prefix:	ctsh_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Line of Credit Facility, Maximum Period Drawn Notes Have Been Outstanding

+ References

No definition available.

+ Details

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Namespace Prefix:	ctsh_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

+ Details

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

- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

+ Details

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

- 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

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

- 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

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

- Details

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Namespace Prefix:
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- Details

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- Details

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- Details

Name:	us-gaap_VariableRateAxis=us-gaap_EurodollarMember
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- Details

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- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
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- Details

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

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
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