Summary of Significant Accounting Policies (Schedule of Effect of New Accounting Pronouncements) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
Revenue:
Perpetual license	$ 683	$ 1,746	$ 2,309	$ 2,440
Operating expenses:
Sales and marketing	5,248	$ 4,037	9,656	$ 7,258
Assets
Deferred commissions, current	2,185		2,185		$ 2,070	$ 0
Deferred commissions, noncurrent	5,023		5,023		4,447	0
Liabilities [Abstract]
Deferred revenue	44,388		44,388		44,032	43,807
Equity [Abstract]
Accumulated deficit	(83,227)		(83,227)		(74,836)	(81,128)
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Revenue:
Perpetual license	752		2,059
Operating expenses:
Sales and marketing	5,610		10,239
Assets
Deferred commissions, current	(239)		(239)			0
Deferred commissions, noncurrent	347		347			0
Liabilities [Abstract]
Deferred revenue	44,140		44,140			43,807
Equity [Abstract]
Accumulated deficit	(88,686)		(88,686)			$ (81,128)
Accounting Standards Update 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Revenue:
Perpetual license	(69)		250
Operating expenses:
Sales and marketing	(362)		(583)
Assets
Deferred commissions, current	2,424		2,424		2,070
Deferred commissions, noncurrent	4,676		4,676		4,447
Liabilities [Abstract]
Deferred revenue	(248)		(248)		225
Equity [Abstract]
Accumulated deficit	$ (5,459)		$ (5,459)		$ 6,292