Consolidated Financial Statements' Components - Property and Equipment, net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 105,820	$ 108,914
Accumulated depreciation	(97,820)	(98,560)
Total property and equipment, net	$ 8,000	10,354
Minimum
Property, Plant and Equipment [Line Items]
Useful Life (In Years)	1 year
Maximum
Property, Plant and Equipment [Line Items]
Useful Life (In Years)	5 years
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 3,620	4,139
Asset retirement costs
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 1,922	2,021
Asset retirement costs | Minimum
Property, Plant and Equipment [Line Items]
Useful Life (In Years)	1 year
Asset retirement costs | Maximum
Property, Plant and Equipment [Line Items]
Useful Life (In Years)	5 years
Paging and computer equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 96,562	98,401
Paging and computer equipment | Minimum
Property, Plant and Equipment [Line Items]
Useful Life (In Years)	1 year
Paging and computer equipment | Maximum
Property, Plant and Equipment [Line Items]
Useful Life (In Years)	5 years
Furniture, fixtures and vehicles
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 3,716	$ 4,353
Furniture, fixtures and vehicles | Minimum
Property, Plant and Equipment [Line Items]
Useful Life (In Years)	3 years
Furniture, fixtures and vehicles | Maximum
Property, Plant and Equipment [Line Items]
Useful Life (In Years)	5 years