Revenue Recognition (Costs to Obtain a Contract) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Disaggregation of Revenue [Line Items]
Deferred commissions costs	$ 8,349	$ 8,349		$ 0
Minimum
Disaggregation of Revenue [Line Items]
Amortization period		3 years
Maximum
Disaggregation of Revenue [Line Items]
Amortization period		7 years
Accounting Standards Update 2014-09
Disaggregation of Revenue [Line Items]
Deferred commissions costs	12,800	$ 12,800	$ 11,800
Commissions expense recognized	$ 1,000	$ 2,000