Consolidated Financial Statement Components - Property and Equipment, Net (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Accounts receivable allowance	$ 1,400	$ 1,100
Total property and equipment	114,169	115,400
Accumulated depreciation	(101,547)	(102,001)
Total property and equipment, net	12,622	13,399
Leasehold improvements
Property, Plant and Equipment [Line Items]
Total property and equipment	4,135	4,107
Asset retirement costs
Property, Plant and Equipment [Line Items]
Total property and equipment	$ 3,243	3,228
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	$ 102,221	103,520
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	$ 4,570	4,545
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
Contract asset
Property, Plant and Equipment [Line Items]
Accounts receivable allowance	$ 10,500	$ 7,300