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Commitments and Contingencies - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2021
Minimum [Member]
Commitments And Contingencies [Line Items]
Number of months of salary paid as severance	3 months
Maximum [Member]
Commitments And Contingencies [Line Items]
Number of months of salary paid as severance	24 months

X

- Definition

Commitments and contingencies.

+ References

No definition available.

+ Details

Name:	nsit_CommitmentsAndContingenciesLineItems
Namespace Prefix:	nsit_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The number of months of salary that would be payable to certain officers and management teammates, if severance payments under current employment agreements or plan payments were to become payable.

+ References

No definition available.

+ Details

Name:	nsit_NumberOfMonthOfSalaryPaidAsSeveranceOnChangeOfControl
Namespace Prefix:	nsit_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Details

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

X

- Details

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