Consolidated Financial Statements' Components - Depreciation, Amortization and Accretion (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Depreciation	$ 7,727	$ 8,176	$ 8,180
Amortization	2,500	2,886	4,160
Accretion	542	562	623
Total depreciation, amortization and accretion expense	10,769	11,624	12,963
Leasehold improvements
Property, Plant and Equipment [Line Items]
Depreciation	232	234	189
Asset retirement costs
Property, Plant and Equipment [Line Items]
Depreciation	(300)	(388)	(277)
Paging and computer equipment
Property, Plant and Equipment [Line Items]
Depreciation	7,397	8,024	7,974
Furniture, fixtures and vehicles
Property, Plant and Equipment [Line Items]
Depreciation	$ 398	$ 306	$ 294