Organization and Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) account message	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Property, Plant and Equipment [Line Items]
Monthly messages sent through company solutions (over 100 million messages) | message	100,000,000
Other current liabilities	$ 4,226,000	$ 3,610,000
General and administrative	47,400,000	42,827,000	$ 44,860,000
Impairment of intangible assets	$ 0	0	0
Number of significant allowance accounts | account	2
Allowances for doubtful accounts	$ 1,000,000	1,000,000
Contractor fee, annual percentage increase	4.00%
Commissions expense	$ 5,200,000	5,600,000	7,200,000
Advertising expenses	2,300,000	1,800,000	1,700,000
Provisions for doubtful accounts, service credits and other	$ 1,035,000	761,000	1,290,000
Minimum
Property, Plant and Equipment [Line Items]
Amortization of intangible assets	6 years
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
Classification of Research and Development Costs
Property, Plant and Equipment [Line Items]
Other current liabilities	$ 1,000,000	900,000
Accounts payable	1,000,000	900,000
General and administrative	100,000	1,400,000	2,700,000
Severance costs	$ 100,000	$ 1,400,000	$ 2,700,000