Consolidated Financial Statement Components - Property and Equipment, net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 115,400	$ 123,133
Accumulated depreciation	(102,001)	(110,315)
Total property and equipment, net	$ 13,399	12,818
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	$ 4,107	3,843
Asset retirement costs
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 3,228	3,263
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	$ 103,520	113,175
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	$ 4,545	$ 2,852
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