Fair Value Measurements - Schedule of Cash and Available-for-sale Debt Securities' Amortized Cost, Unrealized Gains (Losses) and Fair Value by Significant Investment Category (Details) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized Cost	$ 109,063,000	$ 138,976,000
Amortized Cost	595,218,000	643,471,000
Unrealized Gain	1,939,000	190,000
Unrealized (Loss)	(456,000)	(53,000)
Fair Value	596,701,000	643,608,000
Cash
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized Cost	6,398,000	24,631,000
Unrealized Gain	0	0
Unrealized (Loss)	0	0
Fair Value	6,398,000	24,631,000
Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Unrealized Gain	0	0
Unrealized (Loss)	0	0
Level I \| Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized Cost	62,449,000	32,856,000
Unrealized Gain	0	0
Unrealized (Loss)	0	0
Fair Value	62,449,000	32,856,000
Level II
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized Cost	526,371,000	585,984,000
Unrealized Gain	1,939,000	190,000
Unrealized (Loss)	(456,000)	(53,000)
Fair Value	527,854,000	586,121,000
Level II \| Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized Cost	116,125,000	84,054,000
Unrealized Gain	12,000	22,000
Unrealized (Loss)	(456,000)	(30,000)
Fair Value	115,681,000	84,046,000
Level II \| U.S. treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized Cost	282,330,000	311,083,000
Unrealized Gain	1,831,000	151,000
Unrealized (Loss)	0	(23,000)
Fair Value	284,161,000	311,211,000
Level II \| U.S. government agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized Cost	55,442,000	95,380,000
Unrealized Gain	96,000	17,000
Unrealized (Loss)	0	0
Fair Value	55,538,000	95,397,000
Level II \| Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized Cost	72,474,000	95,467,000
Unrealized Gain	0	0
Unrealized (Loss)	0	0
Fair Value	72,474,000	95,467,000
Cash & Cash Equivalents \| Fair Value, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	109,063,000	138,976,000
Cash & Cash Equivalents \| Fair Value, Recurring \| Cash
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	4,211,000	24,631,000
Cash & Cash Equivalents \| Fair Value, Recurring \| Level I \| Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	55,789,000	26,196,000
Cash & Cash Equivalents \| Fair Value, Recurring \| Level II
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	49,063,000	88,149,000
Cash & Cash Equivalents \| Fair Value, Recurring \| Level II \| Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	0
Cash & Cash Equivalents \| Fair Value, Recurring \| Level II \| U.S. treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	0
Cash & Cash Equivalents \| Fair Value, Recurring \| Level II \| U.S. government agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	10,000,000	22,549,000
Cash & Cash Equivalents \| Fair Value, Recurring \| Level II \| Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	39,063,000	65,600,000
Available-for-sale securities \| Fair Value, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	478,791,000	497,972,000
Available-for-sale securities \| Fair Value, Recurring \| Cash
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	0
Available-for-sale securities \| Fair Value, Recurring \| Level I \| Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	0
Available-for-sale securities \| Fair Value, Recurring \| Level II
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	478,791,000	497,972,000
Available-for-sale securities \| Fair Value, Recurring \| Level II \| Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	115,681,000	84,046,000
Available-for-sale securities \| Fair Value, Recurring \| Level II \| U.S. treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	284,161,000	311,211,000
Available-for-sale securities \| Fair Value, Recurring \| Level II \| U.S. government agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	45,538,000	72,848,000
Available-for-sale securities \| Fair Value, Recurring \| Level II \| Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	33,411,000	29,867,000
Long-term Restricted Cash \| Fair Value, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	8,847,000	6,660,000
Long-term Restricted Cash \| Fair Value, Recurring \| Cash
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	2,187,000	0
Long-term Restricted Cash \| Fair Value, Recurring \| Level I \| Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	6,660,000	6,660,000
Long-term Restricted Cash \| Fair Value, Recurring \| Level II
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	0
Long-term Restricted Cash \| Fair Value, Recurring \| Level II \| Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	0
Long-term Restricted Cash \| Fair Value, Recurring \| Level II \| U.S. treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	0
Long-term Restricted Cash \| Fair Value, Recurring \| Level II \| U.S. government agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	0
Long-term Restricted Cash \| Fair Value, Recurring \| Level II \| Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	$ 0	$ 0

Cash And Cash Equivalents, Accumulated Gross Unrealized Gain, Before Tax

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Cash And Cash Equivalents, Accumulated Gross Unrealized Loss, Before Tax

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Cash And Cash Equivalents And Debt Securities, Available-For-Sale

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Cash And Cash Equivalents And Debt Securities, Available-For-Sale, Amortized Cost

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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 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 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. Excludes cash and cash equivalents within disposal group and discontinued operation.

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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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