Organization and Significant Accounting Policies - Additional Information (Details) message in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) account message	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Property, Plant and Equipment [Line Items]
Monthly messages sent through company solutions (over 100 million messages) | message									100
Revenues	$ 43,256,000	$ 42,476,000	$ 40,628,000	$ 43,114,000	$ 43,770,000	$ 43,636,000	$ 42,325,000	$ 41,444,000	$ 169,474,000	$ 171,175,000	$ 179,561,000
Decrease to cash and cash equivalents	(83,343,000)				(103,179,000)				(83,343,000)	(103,179,000)	(121,825,000)	$ (109,335,000)
Short-term investments	3,963,000				3,978,000				3,963,000	3,978,000
Decrease to accounts receivable, net	(32,386,000)				(29,722,000)				(32,386,000)	(29,722,000)
Decrease to deferred revenue	(26,761,000)				(29,920,000)				(26,761,000)	(29,920,000)
Decrease to current assets	(130,978,000)				(144,303,000)				(130,978,000)	(144,303,000)
Decrease to current liabilities	(44,948,000)				(48,337,000)				(44,948,000)	(48,337,000)
Decrease to total assets	(327,712,000)				(348,004,000)				(327,712,000)	(348,004,000)
Decrease to total liabilities	(53,158,000)				(57,475,000)				(53,158,000)	(57,475,000)
Decrease to total liabilities and stockholders' equity	(327,712,000)				(348,004,000)				(327,712,000)	(348,004,000)
Commissions expense									6,200,000	5,200,000	5,600,000
Impairment of intangible assets									$ 0	0	0
Number of significant allowance accounts | account									2
Contractor fee, annual percentage increase									2.00%
Uncertain tax positions	0				0				$ 0	0
Advertising expenses									2,400,000	2,300,000	1,800,000
Provisions for doubtful accounts, service credits and other									$ 1,600,000	500,000	400,000
Minimum
Property, Plant and Equipment [Line Items]
Useful life									1 year
Maximum
Property, Plant and Equipment [Line Items]
Amortization of intangible assets									10 years
Useful life									5 years
Paging Equipment Assets | Minimum
Property, Plant and Equipment [Line Items]
Useful life									1 year
Paging Equipment Assets | Maximum
Property, Plant and Equipment [Line Items]
Useful life									5 years
Accounting Standards Update 2014-09
Property, Plant and Equipment [Line Items]
Decrease to deferred revenue					(2,600,000)					(2,600,000)
Accounting Standards Update 2014-09 | Licenses
Property, Plant and Equipment [Line Items]
Revenues	2,900,000									2,300,000	2,100,000
Accounting Standards Update 2014-09 | Subscription Revenue
Property, Plant and Equipment [Line Items]
Revenues	(2,900,000)									(2,300,000)	$ (2,100,000)
Adjustments
Property, Plant and Equipment [Line Items]
Decrease to cash and cash equivalents	4,000,000				4,000,000				$ 4,000,000	4,000,000
Short-term investments	4,000,000				4,000,000				4,000,000	4,000,000
Decrease to accounts receivable, net	2,600,000				2,600,000				2,600,000	2,600,000
Decrease to deferred revenue	$ 2,600,000				2,600,000				$ 2,600,000	2,600,000
Decrease to current assets					2,600,000					2,600,000
Decrease to current liabilities					2,600,000					2,600,000
Decrease to total assets					2,600,000					2,600,000
Decrease to total liabilities					2,600,000					2,600,000
Decrease to total liabilities and stockholders' equity					$ 2,600,000					$ 2,600,000