Consolidated Financial Statement Components - Depreciation, Amortization and Accretion (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Property, Plant and Equipment [Line Items]
Depreciation	$ 8,176	$ 8,180	$ 8,570
Amortization	2,886	4,160	4,735
Accretion	562	623	665
Total depreciation, amortization and accretion expense	11,624	12,963	13,970
Leasehold improvements
Property, Plant and Equipment [Line Items]
Depreciation	234	189	233
Paging and computer equipment
Property, Plant and Equipment [Line Items]
Depreciation	8,024	7,974	8,489
Furniture, fixtures and vehicles
Property, Plant and Equipment [Line Items]
Depreciation	306	294	353
Depreciable life | Asset retirement costs
Property, Plant and Equipment [Line Items]
Depreciation	$ (388)	$ (277)	$ (505)