Investments (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Certificates of deposit	$ 6,000	$ 2,880
Certificates of deposit interest rate, lower range	1.00%
Certificates of deposit interest rate, upper range	1.60%
Investments held to maturity	$ 8,942	$ 6,077
Minimum
Schedule of Investments [Line Items]
Certificates of deposit maturity dates	1 month
Investments maturity dates	1 month
Maximum
Schedule of Investments [Line Items]
Certificates of deposit maturity dates	4 months
Investments maturity dates	4 months