Accounts Receivable, Net - Schedule of Credit Losses (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounting Standards Update [Extensible List]			us-gaap:AccountingStandardsUpdate201613Member		us-gaap:AccountingStandardsUpdate201613Member	us-gaap:AccountingStandardsUpdate201602Member
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning of period balance			$ (2,584)
Recovery of expected credit losses	$ 1,200	$ (200)	237	$ (722)
End of period balance	(2,400)		(2,400)		$ (2,584)
Alarm.com and Certain Subsidiaries
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning of period balance	(3,550)		(2,500)
Recovery of expected credit losses	1,146		195
Write-offs	170		283
End of period balance	(2,234)		(2,234)		(2,500)
Alarm.com and Certain Subsidiaries | Impact of adopting Topic 326
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning of period balance	0		(212)
End of period balance					(212)
All Other Subsidiaries
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning of period balance	(238)		(84)
Recovery of expected credit losses	56		42
Write-offs	16		31
End of period balance	(166)		(166)		(84)
All Other Subsidiaries | Impact of adopting Topic 326
Accounts Receivable, Allowance for Credit Loss [Roll Forward]
Beginning of period balance	$ 0		$ (155)
End of period balance					$ (155)