Commitments and Contingencies - Additional Information (Detail)	9 Months Ended
Sep. 30, 2020
Commitments And Contingencies Disclosure [Abstract]
Number of months of salary paid as severance	from three to twenty-four months

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.

No definition available.

Name:	nsit_NumberOfMonthsOfSalaryPaidAsSeveranceOnChangeOfControl
Namespace Prefix:	nsit_
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	duration