Consolidated Financial Statements' Components - Property and Equipment, net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 108,914	$ 115,400
Accumulated depreciation	(98,560)	(102,001)
Total property and equipment, net	$ 10,354	13,399
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	$ 4,139	4,107
Asset retirement costs
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 2,021	3,228
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	$ 98,401	103,520
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	$ 4,353	$ 4,545
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