Consolidated Financial Statement Components - Property and Equipment, Net (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 108,460	$ 108,914
Accumulated depreciation	(98,701)	(98,560)
Total property and equipment, net	9,759	10,354
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	3,619	4,139
Asset retirement costs
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 2,036	2,021
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	$ 98,452	98,401
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	$ 4,353	$ 4,353
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