Summary of Significant Accounting Policies (Schedule of Cumulative Effect of ASU 2014-09 Adoption on Balance Sheet) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets
Deferred commissions, current	$ 2,185	$ 2,070	$ 0
Deferred commissions, noncurrent	5,023	4,447	0
Liabilities [Abstract]
Deferred revenue	44,388	44,032	43,807
Equity [Abstract]
Accumulated deficit	(83,227)	(74,836)	(81,128)
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Assets
Deferred commissions, current	(239)		0
Deferred commissions, noncurrent	347		0
Liabilities [Abstract]
Deferred revenue	44,140		43,807
Equity [Abstract]
Accumulated deficit	(88,686)		$ (81,128)
Accounting Standards Update 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Assets
Deferred commissions, current	2,424	2,070
Deferred commissions, noncurrent	4,676	4,447
Liabilities [Abstract]
Deferred revenue	(248)	225
Equity [Abstract]
Accumulated deficit	$ (5,459)	$ 6,292