Stock Options and Employee Stock Purchase Plan - Summarized Information Concerning Outstanding and Exercisable Options (Details)	12 Months Ended
Dec. 31, 2020 $ / shares shares
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Number outstanding (in shares) \| shares	475,601
Weighted average remaining contractual life	6 years 1 month 24 days
Options outstanding, weighted average exercise price (in usd per share)	$ 69.61
Number exercisable (in shares) \| shares	175,601
Exercisable options, weighted average exercise price (in usd per share)	$ 60.35
Range One
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Rnage of exercise price (in usd per share)	$ 29.34
Number outstanding (in shares) \| shares	45,351
Weighted average remaining contractual life	4 months 9 days
Options outstanding, weighted average exercise price (in usd per share)	$ 29.34
Number exercisable (in shares) \| shares	45,351
Exercisable options, weighted average exercise price (in usd per share)	$ 29.34
Range Two
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Rnage of exercise price (in usd per share)	$ 29.53
Number outstanding (in shares) \| shares	7,250
Weighted average remaining contractual life	1 year 2 months 1 day
Options outstanding, weighted average exercise price (in usd per share)	$ 29.53
Number exercisable (in shares) \| shares	7,250
Exercisable options, weighted average exercise price (in usd per share)	$ 29.53
Range Three
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Rnage of exercise price (in usd per share)	$ 67.35
Number outstanding (in shares) \| shares	23,000
Weighted average remaining contractual life	4 years 4 months 6 days
Options outstanding, weighted average exercise price (in usd per share)	$ 67.35
Number exercisable (in shares) \| shares	23,000
Exercisable options, weighted average exercise price (in usd per share)	$ 67.35
Range Four
Share-based Payment Arrangement, Option, Exercise Price Range [Line Items]
Rnage of exercise price (in usd per share)	$ 75.03
Number outstanding (in shares) \| shares	400,000
Weighted average remaining contractual life	7 years
Options outstanding, weighted average exercise price (in usd per share)	$ 75.03
Number exercisable (in shares) \| shares	100,000
Exercisable options, weighted average exercise price (in usd per share)	$ 75.03

Share based compensation shares authorized under stock option plans exercise price range.

No definition available.

Name:	jcom_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRange
Namespace Prefix:	jcom_
Data Type:	num:perShareItemType
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_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of shares reserved for issuance pertaining to the outstanding exercisable stock options as of the balance sheet date in the customized range of exercise prices for which the market and performance vesting condition has been satisfied.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(iii)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of shares reserved for issuance pertaining to the outstanding stock options as of the balance sheet date for all option plans in the customized range of exercise prices.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Weighted average exercise price as of the balance sheet date for those equity-based payment arrangements exercisable and outstanding.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iii)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageExercisePrice1
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

The weighted average price as of the balance sheet date at which grantees could acquire the underlying shares with respect to all outstanding stock options which are in the customized range of exercise prices.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageExercisePriceBeginningBalance1
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Weighted average remaining contractual term of outstanding stock options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (e)(1)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageRemainingContractualTerm2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=jcom_RangeOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=jcom_RangeTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=jcom_RangeThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=jcom_RangeFourMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: