Business Combinations, Strategic Partnerships, and Asset Acquisitions - Narrative (Details)		1 Months Ended		12 Months Ended
	Jan. 14, 2019 USD ($)	Jul. 31, 2020 USD ($)	Oct. 31, 2019 USD ($) $ / shares	Dec. 31, 2019 USD ($)	Sep. 30, 2020 USD ($)
Business Acquisition [Line Items]
Goodwill				$ 55,278,000	$ 56,223,000
Long-term investments				132,188,000	$ 173,641,000
Acquired intangible assets				105,500,000
Deferred sales commissions, noncurrent				$ 371,100,000
Strategic partnership agreement, fee		$ 100,000,000.0
Minimum
Business Acquisition [Line Items]
Weighted average amortization periods			3 years
Maximum
Business Acquisition [Line Items]
Weighted average amortization periods			5 years
Avaya
Business Acquisition [Line Items]
Long-term investments			$ 125,000,000.0
Preferred stock, initial conversion price (in dollars per share) | $ / shares			$ 16.00
Percentage interests acquired in Avaya upon conversion (percentage)			6.00%
Consideration to be transferred			$ 345,000,000.0
Avaya | Avaya
Business Acquisition [Line Items]
Preferred stock, dividend rate (percentage)			0.03
Connect First, Inc.
Business Acquisition [Line Items]
Total purchase consideration	$ 36,400,000
Cash payments	29,300,000
Contingent consideration	$ 7,100,000
Estimated lives	3 years
Goodwill	$ 24,465,000
Amount of goodwill expected to be tax deductible	0
Connect First, Inc. | Restricted stock units
Business Acquisition [Line Items]
Consideration, restricted stock unit	$ 4,000,000.0
Consideration, restricted stock unit, vesting period	4 years