General (Details Textual)				3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) subsidiary	Dec. 31, 2018	Mar. 31, 2014 USD ($)	Mar. 31, 2019 USD ($) subsidiary	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)	Jan. 01, 2019 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operations commenced date				Jan. 01, 2005
Subsidiary or equity method investee, number | subsidiary	9			9
Entity incorporation, date of incorporation				Nov. 24, 2004
Deferred revenue, revenue recognized				$ 32,061
Sales returns				0		$ 0
Gain in financial income, net				471,000	$ 0
Other than temporary impairment losses, investments, available-for-sale securities				0
Operating lease right-of-use asset	$ 51,875,000			51,875,000			$ 53,274,000
Operating lease liability, current	3,468,000			3,468,000
Operating lease liabilities long-term	$ 52,697,000			52,697,000
Foreign Exchange Contract
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Derivative, gain (loss) on derivative, net, total				(451,000)	48,000
Material ineffective hedges recognized				$ 0	$ 0
US Treasury Securities | Maximum | Short-term investments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Short-term Investment, maturity period	1 year	1 year
Promissory Note
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Line of credit facility, maximum borrowing capacity			$ 7,000,000
Basis spread on variable rate (percentage)			0.05%
Interest rate at period end	5.55%			5.55%
Long-term line of credit	$ 0			$ 0
Promissory Note | Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Interest rate, effective percentage			4.10%
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right-of-use asset							53,274,000
Operating lease liability, current							2,349,000
Operating lease liabilities long-term							55,676,000
Liabilities, Current | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Elimination of deferred rent							1,313,000
Liabilities, Noncurrent | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Elimination of deferred rent							$ 4,236,000