Goodwill and Intangible Assets (Changes in Carrying Amounts of Goodwill) (Details) - USD ($) $ in Thousands		9 Months Ended
	Aug. 31, 2020	Sep. 30, 2020
Goodwill [Line Items]
Balance as of January 1, 2020		$ 1,633,033
Goodwill acquired		23,817
Goodwill written off related to sale of a business		(4,751)
Purchase accounting adjustments		7,591
Foreign exchange translation		1,856
Balance as of September 30, 2020		1,661,546
Voice, Backup, Security and CPP
Goodwill [Line Items]
Goodwill written off related to sale of a business	$ (4,800)
Purchase accounting adjustments		(2,100)
Digital Media
Goodwill [Line Items]
Purchase accounting adjustments		9,700
Operating Segments | Total Cloud Services
Goodwill [Line Items]
Balance as of January 1, 2020		877,872
Goodwill acquired		21,950
Goodwill written off related to sale of a business		(4,751)
Purchase accounting adjustments		(2,130)
Foreign exchange translation		1,868
Balance as of September 30, 2020		894,809
Operating Segments | Fax and Martech
Goodwill [Line Items]
Balance as of January 1, 2020		397,788
Goodwill acquired		21,950
Goodwill written off related to sale of a business		0
Purchase accounting adjustments		0
Foreign exchange translation		2,226
Balance as of September 30, 2020		421,964
Operating Segments | Voice, Backup, Security and CPP
Goodwill [Line Items]
Balance as of January 1, 2020		480,084
Goodwill acquired		0
Goodwill written off related to sale of a business		(4,751)
Purchase accounting adjustments		(2,130)
Foreign exchange translation		(358)
Balance as of September 30, 2020		472,845
Operating Segments | Digital Media
Goodwill [Line Items]
Balance as of January 1, 2020		755,161
Goodwill acquired		1,867
Goodwill written off related to sale of a business		0
Purchase accounting adjustments		9,721
Foreign exchange translation		(12)
Balance as of September 30, 2020		$ 766,737