Earnings Per Share - Components of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Nov. 15, 2019	Jun. 10, 2014
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net income attributable to J2 Global, Inc. common shareholders	$ 58,088	$ 60,883	$ 38,101	$ (6,404)	$ 123,023	$ 30,745	$ 32,589	$ 32,449	$ 150,668	$ 218,806	$ 128,687
Net income available to participating securities									(632)	(3,496)	(1,885)
Net income available to J2 Global, Inc. common shareholders									$ 150,036	$ 215,310	$ 126,802
Denominator:
Weighted-average outstanding shares of common stock - basic (in shares)	44,504,222	46,279,515	46,850,944	47,620,774	47,626,833	47,673,211	47,727,786	47,560,749	46,308,825	47,647,397	47,950,746
Dilutive effect of:
Equity incentive plans (in shares)									25,232	78,076	146,906
Convertible debt (in shares)									788,454	1,300,211	830,139
Common stock and common stock equivalents (in shares)	45,642,292	46,309,072	47,437,555	47,620,774	49,425,395	49,064,272	49,102,879	48,509,181	47,122,511	49,025,684	48,927,791
Net income per share:
Basic (in dollars per share)	$ 1.30	$ 1.31	$ 0.81	$ (0.13)	$ 2.54	$ 0.63	$ 0.67	$ 0.67	$ 3.24	$ 4.52	$ 2.64
Diluted (in dollars per share)	$ 1.27	$ 1.31	$ 0.80	$ (0.13)	$ 2.45	$ 0.62	$ 0.66	$ 0.66	$ 3.18	$ 4.39	$ 2.59
3.25% Convertible Notes | Convertible Debt
Net income per share:
Stated interest rate													3.25%
1.75% Convertible Notes | Convertible Debt
Net income per share:
Stated interest rate												1.75%