Fair Value of Financial Instruments - Cash and Cash Equivalents, Available-for-sale Securities Reconciliation (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	$ 75,615
Unrealized Gains	1,182	$ 22
Unrealized (Losses)	(20)	(567)
Fair Value	398,987	365,876
Debt Securities, Available-for-sale, Amortized Cost	397,825	366,421
Cash and cash equivalents:
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure		41,026
Current Assets [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	306	9
Unrealized (Losses)	(14)	(193)
Fair Value	222,421	248,140
Debt Securities, Available-for-sale, Amortized Cost	222,129	248,324
Long-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	876	13
Unrealized (Losses)	(6)	(374)
Fair Value	100,951	76,710
Debt Securities, Available-for-sale, Amortized Cost	100,081	77,071
Asset backed securities \| Current Assets [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	1	0
Unrealized (Losses)	0	(15)
Fair Value	1,052	10,432
Debt Securities, Available-for-sale, Amortized Cost	1,051	10,447
Asset backed securities \| Long-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	252	10
Unrealized (Losses)	0	(28)
Fair Value	40,083	22,927
Debt Securities, Available-for-sale, Amortized Cost	39,831	22,945
Commercial paper \| Cash and cash equivalents:
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Fair Value	8,913
Available-for-sale Securities, Amortized Cost Basis	8,913
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0
Commercial paper \| Short-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Fair Value	3,386
Available-for-sale Securities, Amortized Cost Basis	3,386
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0
Commercial paper \| Current Assets [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains		0
Unrealized (Losses)		0
Fair Value		3,237
Debt Securities, Available-for-sale, Amortized Cost		3,237
Corporate bonds \| Current Assets [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	70	0
Unrealized (Losses)	(9)	(84)
Fair Value	30,099	30,822
Debt Securities, Available-for-sale, Amortized Cost	30,038	30,906
Corporate bonds \| Long-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	242	3
Unrealized (Losses)	(6)	(293)
Fair Value	32,612	35,032
Debt Securities, Available-for-sale, Amortized Cost	32,376	35,322
US government agencies \| Current Assets [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	235	9
Unrealized (Losses)	(5)	(94)
Fair Value	187,884	203,649
Debt Securities, Available-for-sale, Amortized Cost	187,654	203,734
US government agencies \| Long-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	382	0
Unrealized (Losses)	0	(53)
Fair Value	28,256	18,751
Debt Securities, Available-for-sale, Amortized Cost	27,874	18,804
Cash Equivalents [Member] \| Money market funds
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	249	113
Cash [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	$ 66,453	$ 40,913

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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