Fair Value of Financial Instruments - Cash and Cash Equivalents, Available-for-sale Securities Reconciliation (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
amortized cost [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	$ 113,909	$ 87,559
Cash and Cash Equivalent and ST and LT marketable securities	442,507	417,332
Fair Value, Recurring [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalent and ST and LT marketable securities	445,314	418,398
Unrealized gains - Cash and Cash Equivalent and ST and LT marketable securities	2,812	1,071
Unrealized losses - Cash and Cash Equivalent and ST and LT marketable securities	(5)	(5)
Cash and cash equivalents: \| Fair Value, Recurring [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	113,909	87,559
Cash and cash equivalents:
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Short-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	861	246
Unrealized (Losses)	(5)	(4)
Fair Value	233,922	211,331
Debt Securities, Available-for-sale, Amortized Cost	233,066	211,089
Long-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	1,951	825
Unrealized (Losses)	0	(1)
Fair Value	97,483	119,508
Debt Securities, Available-for-sale, Amortized Cost	95,532	118,684
Asset backed securities \| Short-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	52	11
Unrealized (Losses)	0	0
Fair Value	6,563	2,449
Debt Securities, Available-for-sale, Amortized Cost	6,511	2,438
Asset backed securities \| Long-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	407	193
Unrealized (Losses)	0	(1)
Fair Value	35,848	40,193
Debt Securities, Available-for-sale, Amortized Cost	35,441	40,001
Corporate bonds \| Short-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	227	51
Unrealized (Losses)	0	(1)
Fair Value	24,516	33,098
Debt Securities, Available-for-sale, Amortized Cost	24,289	33,048
Corporate bonds \| Long-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	1,066	262
Unrealized (Losses)	0	0
Fair Value	45,608	32,498
Debt Securities, Available-for-sale, Amortized Cost	44,542	32,236
US government agencies \| Short-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	580	184
Unrealized (Losses)	(5)	(3)
Fair Value	200,994	173,545
Debt Securities, Available-for-sale, Amortized Cost	200,419	173,364
US government agencies \| Long-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	444	370
Unrealized (Losses)	0	0
Fair Value	14,988	46,817
Debt Securities, Available-for-sale, Amortized Cost	14,544	46,447
Commercial Paper [Member] \| Cash and cash equivalents:
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	0	0
Available-for-sale Securities, Accumulated Gross Unrealized Loss, before Tax	0	0
Commercial Paper [Member] \| Short-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized (Losses)	0	0
Fair Value	1,849	2,239
Debt Securities, Available-for-sale, Amortized Cost	1,847	2,239
Available-for-sale Securities, Accumulated Gross Unrealized Gain, before Tax	2	0
Sovereign Debt Securities \| Long-term investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	34
Unrealized (Losses)	0
Fair Value	1,039
Debt Securities, Available-for-sale, Amortized Cost	1,005
Cash [Member] \| amortized cost [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	106,272	84,102
Cash [Member] \| Fair Value, Recurring [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	106,272	84,102
Money market funds \| amortized cost [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	5,638	58
Money market funds \| Fair Value, Recurring [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	5,638	58
Commercial Paper [Member] \| amortized cost [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	1,999	3,399
Commercial Paper [Member] \| Fair Value, Recurring [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	$ 1,999	$ 3,399

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Unrealized gains - Cash and Cash Equivalent and ST and LT marketable securities

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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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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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Amortized cost of 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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