Marketable Securities - Available-for-Sale (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized cost, current	$ 401	$ 562
Accumulated gross unrealized gains, current	11,394	9,388
Accumulated gross unrealized losses, current	0	(7)
Estimated fair value, current	11,795	9,943
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost, current	1	112
Accumulated gross unrealized gains, current	0	47
Accumulated gross unrealized losses, current	0	(7)
Estimated fair value, current	1	152
Mortgage securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost, current	400	450
Accumulated gross unrealized gains, current	11,394	9,341
Accumulated gross unrealized losses, current	0	0
Estimated fair value, current	$ 11,794	9,791
Agency mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost, noncurrent		26,607
Accumulated gross unrealized gains, noncurrent		0
Accumulated gross unrealized losses, noncurrent		(62)
Estimated fair value, noncurrent		$ 26,545

Available-for-sale Securities, Current, Accumulated Gross Unrealized Gains

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Available-for-sale Securities, Current, Accumulated Gross Unrealized Losses

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Available-for-sale Securities, Current, Amortized Cost Basis

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Available-for-sale Securities, Noncurrent, Accumulated Gross Unrealized Gains

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Amount of investment in debt and equity securities categorized neither as trading securities nor held-to-maturity securities and intended be sold or mature one year or operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26626-111562

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Investments in debt and equity securities which are categorized neither as held-to-maturity nor trading and which are intended to be sold or mature more than one year from the balance sheet date or operating cycle, if longer. Such securities are reported at fair value; unrealized gains (losses) related to Available-for-sale Securities are excluded from earnings and reported in a separate component of shareholders' equity (other comprehensive income), unless the Available-for-sale security is designated as a hedge or is determined to have had an other than temporary decline in fair value below its amortized cost basis. All or a portion of the unrealized holding gain (loss) of an Available-for-sale security that is designated as being hedged in a fair value hedge is recognized in earnings during the period of the hedge, as are other than temporary declines in fair value below the cost basis for investments in equity securities and debt securities that an entity intends to sell or it is more likely than not that it will be required to sell before the recovery of its amortized cost basis. Other than temporary declines in fair value below the cost basis for debt securities categorized as Available-for-sale that an entity does not intend to sell and for which it is not more likely than not that the entity will be required to sell before the recovery of its amortized cost basis are bifurcated into credit losses and losses related to all other factors. Other than temporary declines in fair value below cost basis related to credit losses are recognized in earnings, and losses related to all other factors are recognized in other comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26626-111562

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