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, 2021	Dec. 31, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized Cost	$ 187,476,000	$ 108,895,000
Amortized Cost	1,044,433,000	1,041,108,000
Unrealized Gain	197,000	270,000
Unrealized (Loss)	(87,000)	(31,000)
Fair Value	1,044,543,000	1,041,347,000
Cash
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized Cost	42,723,000	22,114,000
Unrealized Gain	0	0
Unrealized (Loss)	0	0
Fair Value	42,723,000	22,114,000
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	154,072,000	71,038,000
Unrealized Gain	0	0
Unrealized (Loss)	0	0
Fair Value	154,072,000	71,038,000
Level II
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized Cost	847,638,000	947,956,000
Unrealized Gain	197,000	270,000
Unrealized (Loss)	(87,000)	(31,000)
Fair Value	847,748,000	948,195,000
Level II \| Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized Cost	151,162,000	169,324,000
Unrealized Gain	17,000	43,000
Unrealized (Loss)	(75,000)	(26,000)
Fair Value	151,104,000	169,341,000
Level II \| U.S. treasury securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized Cost	501,018,000	576,652,000
Unrealized Gain	174,000	223,000
Unrealized (Loss)	(12,000)	(4,000)
Fair Value	501,180,000	576,871,000
Level II \| U.S. government agency securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized Cost	15,565,000	15,617,000
Unrealized Gain	6,000	4,000
Unrealized (Loss)	0	(1,000)
Fair Value	15,571,000	15,620,000
Level II \| Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized Cost	179,893,000	186,363,000
Unrealized Gain	0	0
Unrealized (Loss)	0	0
Fair Value	179,893,000	186,363,000
Cash & Cash Equivalents \| Fair Value, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	187,476,000	108,895,000
Cash & Cash Equivalents \| Fair Value, Recurring \| Cash
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	40,064,000	19,523,000
Cash & Cash Equivalents \| Fair Value, Recurring \| Level I \| Money market funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	147,412,000	64,378,000
Cash & Cash Equivalents \| Fair Value, Recurring \| Level II
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	24,994,000
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	0	0
Cash & Cash Equivalents \| Fair Value, Recurring \| Level II \| Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	24,994,000
Available-for-sale securities \| Fair Value, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	847,748,000	923,201,000
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	847,748,000	923,201,000
Available-for-sale securities \| Fair Value, Recurring \| Level II \| Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	151,104,000	169,341,000
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	501,180,000	576,871,000
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	15,571,000	15,620,000
Available-for-sale securities \| Fair Value, Recurring \| Level II \| Commercial paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	179,893,000	161,369,000
Restricted Cash \| Fair Value, Recurring
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	9,319,000	9,251,000
Restricted Cash \| Fair Value, Recurring \| Cash
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	2,659,000	2,591,000
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
Restricted Cash \| Fair Value, Recurring \| Level II
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	0
Restricted Cash \| Fair Value, Recurring \| Level II \| Corporate bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value	0	0
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
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
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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