Acquisitions and Dispositiions - Assets Acquired and Liabilities Assumed - New Voice Media (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Oct. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Accrued Expenses	$ 333
Business Combination Provisional Information, Initial Accounting Incomplete Adjustment, Deferred Tax Liabilities	(598)
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Deferred Revenue	800
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Liabilities	535
Goodwill	602,970	$ 598,499		$ 373,764
New Voice Media
Business Acquisition [Line Items]
Cash and cash equivalents	1,994		$ 1,994
Accounts receivable	12,299		13,747
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Accounts Receivable	(1,448)
Other current assets	3,342		3,907
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Other Assets	(565)
Property and equipment	3,474		3,474
Intangible assets	154,300		154,300
Other assets	378		378
Total assets acquired	175,787		177,800
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustment, Assets	(2,013)
Accounts payable	4,712		4,712
Accrued expenses	4,478		4,145
Deferred tax liabilities, net, non-current	7,158		7,756
Deferred revenue	22,800		22,000
Total liabilities assumed	39,148		38,613
Net identifiable assets acquired	136,639		139,187
Business Combination Provisional Information, Initial Accounting Incomplete Adjustment, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net	(2,548)
Goodwill	213,540		210,992
Business Combination Provisional Information, Initial Accounting Incomplete Adjustment, Goodwill	2,548
Total purchase price	$ 350,179		$ 350,179