Consolidated Financial Statement Components - Property and Equipment, Net (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 104,669	$ 105,820
Accumulated depreciation	(97,014)	(97,820)
Total property and equipment, net	7,655	8,000
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	3,620	3,620
Asset retirement costs
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 1,922	1,922
Asset retirement costs | Minimum
Property, Plant and Equipment [Line Items]
Useful Life	1 year
Asset retirement costs | Maximum
Property, Plant and Equipment [Line Items]
Useful Life	5 years
Paging and computer equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 95,542	96,562
Paging and computer equipment | Minimum
Property, Plant and Equipment [Line Items]
Useful Life	1 year
Paging and computer equipment | Maximum
Property, Plant and Equipment [Line Items]
Useful Life	5 years
Furniture, fixtures and vehicles
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 3,585	$ 3,716
Furniture, fixtures and vehicles | Minimum
Property, Plant and Equipment [Line Items]
Useful Life	3 years
Furniture, fixtures and vehicles | Maximum
Property, Plant and Equipment [Line Items]
Useful Life	5 years