Consolidated Financial Statement Components - Property and Equipment, net (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 123,133	$ 124,303
Accumulated depreciation	(110,315)	(108,917)
Total property and equipment, net	$ 12,818	15,386
Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	1 year
Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 3,843	3,699
Asset retirement costs
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 3,263	3,566
Asset retirement costs | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	1 year
Asset retirement costs | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years
Paging and computer equipment
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 113,175	114,390
Paging and computer equipment | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	1 year
Paging and computer equipment | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years
Furniture, fixtures and vehicles
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 2,852	$ 2,648
Furniture, fixtures and vehicles | Minimum
Property, Plant and Equipment [Line Items]
Estimated useful life	3 years
Furniture, fixtures and vehicles | Maximum
Property, Plant and Equipment [Line Items]
Estimated useful life	5 years