Investments in Available-for-Sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Debt securities, current	$ 155,846	$ 88,072
Total investments, cost basis	156,189	89,910
Total investments, accumulated gross unrealized gains	22	388
Total investments, accumulated gross unrealized losses	(365)	(89)
Total investments, fair value	155,846	90,209
Short-term investments
Schedule of Available-for-sale Securities [Line Items]
Total investments, current	155,846	88,072
Other assets
Schedule of Available-for-sale Securities [Line Items]
Total investments, noncurrent	0	2,137
U.S. government and agency securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, cost basis	64,151
Debt securities, accumulated gross unrealized gains	22
Debt securities, accumulated gross unrealized losses	(73)
Debt securities, fair value	64,100
U.S. government and agency securities \| Short-term investments
Schedule of Available-for-sale Securities [Line Items]
Debt securities, current	64,100
U.S. government and agency securities \| Other assets
Schedule of Available-for-sale Securities [Line Items]
Debt securities, noncurrent		0
Corporate notes and bonds
Schedule of Available-for-sale Securities [Line Items]
Debt securities, cost basis	92,038	88,161
Debt securities, accumulated gross unrealized gains	0	0
Debt securities, accumulated gross unrealized losses	(292)	(89)
Debt securities, fair value	91,746	88,072
Corporate notes and bonds \| Short-term investments
Schedule of Available-for-sale Securities [Line Items]
Debt securities, current	$ 91,746	88,072
Corporate notes and bonds \| Other assets
Schedule of Available-for-sale Securities [Line Items]
Debt securities, noncurrent		0
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities, cost basis		1,749
Debt securities, accumulated gross unrealized gains		388
Debt securities, accumulated gross unrealized losses		0
Debt securities, fair value		2,137
Equity securities \| Short-term investments
Schedule of Available-for-sale Securities [Line Items]
Debt securities, current		0
Equity securities \| Other assets
Schedule of Available-for-sale Securities [Line Items]
Debt securities, noncurrent		$ 2,137

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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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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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.

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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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.

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