Investments - Cost and Carrying Value of Marketable Investments Before Adoption of ASU 2016-01 (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Debt securities: corporate bonds
Cost		$ 700
Gross Unrealized Gains		6
Gross Unrealized Losses		(22)
Carrying Value	$ 654	684
Marketable equity securities
Cost		513
Gross Unrealized Gains		710
Gross Unrealized Losses		(5)
Carrying Value		1,218
Total
Cost		1,213
Gross Unrealized Gains		716
Gross Unrealized Losses		(27)
Carrying Value		$ 1,902

Available-For-Sale Debt Securities [Abstract]

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Available-For-Sale Equity Securities [Abstract]

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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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-Publisher FASB
-Name Accounting Standards Codification
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-Section 50
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in equity securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in equity securities classified as available-for-sale.

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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