Consolidated Financial Statement Components - Property and Equipment, Net (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Accounts receivable allowance	$ 1,100	$ 1,100
Total property and equipment	124,587	123,133
Accumulated depreciation	(110,698)	(110,315)
Total property and equipment, net	13,889	12,818
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	4,051	3,843
Asset retirement costs
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 3,289	3,263
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	$ 112,800	113,175
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	$ 4,447	$ 2,852
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