Available-for-Sale Securities and Fair Value Measurements (Narrative) (Details)	3 Months Ended
	Mar. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Maximum maturity period	2 years
Weighted average maturity period	6 months
Minimum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long term marketable securities, remaining maturity	12 months
Maximum
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long term marketable securities, remaining maturity	18 months