Accumulated Other Comprehensive Income (Reclassification out of Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accumulated Other Comprehensive Income [Abstract]
Total reclassifications for the period	$ 0	$ 0	$ 0

The Total of Other Comprehensive Income Loss, Reclassification Adjustments, Total, Net of Tax.

No definition available.

Name:	jcom_OtherComprehensiveIncomeLossReclassificationAdjustmentsTotalNetOfTax
Namespace Prefix:	jcom_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration