Fair Value of Financial Instruments (Cash and Cash Equivalents, Short-term and Long-term Marketable Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Fair Value, Asset Value Amortized Cost	$ 417,332		$ 366,421
Fair Value, Asset Value, Unrealized Gains	1,071		22
Fair Value, Asset Value, Unrealized Losses	5		567
Assets, Fair Value Disclosure	418,398		365,876
Noncurrent Assets [Member] \| U.S. government agencies
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Debt Securities, Available-for-sale, Amortized Cost	46,447		18,804
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	370		0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0		(53)
Fair value	46,817		18,751
Noncurrent Assets [Member] \| Corporate bonds
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Debt Securities, Available-for-sale, Amortized Cost	32,236		35,322
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	262		3
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0		(293)
Fair value	32,498		35,032
Noncurrent Assets [Member] \| Asset-backed securities
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Debt Securities, Available-for-sale, Amortized Cost	40,001		22,945
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	193		10
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(1)		(28)
Fair value	40,193		22,927
Short-term Investments [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Debt Securities, Available-for-sale, Amortized Cost	211,089
Unrealized Gains	246
Unrealized Losses	(4)
Fair value	211,331
Long-Term Investments
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Debt Securities, Available-for-sale, Amortized Cost	118,684		77,071
Unrealized Gains	825		13
Unrealized Losses	(1)		(374)
Assets, Fair Value Disclosure	119,508
Fair value			76,710
Available-for-sale Securities, Line Item [Member]
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Debt Securities, Available-for-sale, Amortized Cost			248,324
Unrealized Gains			9
Unrealized Losses			(193)
Fair value			248,140
Available-for-sale Securities, Line Item [Member] \| Commercial paper
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Debt Securities, Available-for-sale, Amortized Cost		$ 2,239
Unrealized Gains		0
Unrealized Losses		$ 0
Fair value	2,239
Current Assets [Member] \| U.S. government agencies
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Debt Securities, Available-for-sale, Amortized Cost	173,364		203,734
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	184		9
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(3)		(94)
Fair value	173,545		203,649
Current Assets [Member] \| Commercial paper
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Debt Securities, Available-for-sale, Amortized Cost			3,237
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax			0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax			0
Fair value			3,237
Current Assets [Member] \| Corporate bonds
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Debt Securities, Available-for-sale, Amortized Cost	33,048		30,906
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	51		0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(1)		(84)
Fair value	33,098		30,822
Current Assets [Member] \| Asset-backed securities
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Debt Securities, Available-for-sale, Amortized Cost	2,438		10,447
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	11
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0		(15)
Fair value	2,449		10,432
Cash and Cash Equivalents \| Cash and Cash Equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	87,559		41,026
Cash [Member] \| Cash and Cash Equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	84,102		40,913
Commercial paper \| Cash and Cash Equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	3,399
Commercial paper \| Cash and Cash Equivalents \| Commercial paper
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Unrealized Gains	0
Unrealized Losses	0
Money Market Funds [Member] \| Cash and Cash Equivalents
Financial Instruments, Financial Assets, Balance Sheet Groupings [Abstract]
Cash and Cash Equivalents, Fair Value Disclosure	$ 58		$ 113

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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

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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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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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