Available-for-Sale Securities and Fair Value Measurements (Narrative) (Details) - USD ($)	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Available For Sale Securities And Fair Value Measurements [Abstract]
Net unrealized losses on available-for-sale investments	$ 50,000	$ 3,000	$ (77,000)	$ (14,000)	$ (100,000)
Net unrealized losses on available-for-sale investments, tax impact	(16,000)	$ 0	$ 25,000	$ 0	$ 0
Maximum maturity period			2 years
Weighted average maturity period			4 months
Fair value, assets, Level 1 to Level 2 transfers, amount	0		$ 0
Fair value, assets, Level 2 to Level 1 transfers, amount	$ 0		$ 0

Available for sale securities and fair value measurements.

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Marketable securities maximum original maturity period.

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Amount of transfers of assets measured on a recurring basis out of Level 1 of the fair value hierarchy into Level 2.

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Amount of transfers of assets measured on a recurring basis out of Level 2 of the fair value hierarchy into Level 1.

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Amount after tax, before reclassification adjustments, of unrealized holding gain (loss) on available-for-sale securities.

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Amount of tax expense (benefit) before reclassification adjustments of unrealized holding gain (loss) on available-for-sale securities.

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