Fair Value Measurements. Cash Equivalents and Marketable Securities - Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Cash and cash equivalents	$ 143,228	$ 140,544
Amortized cost, cash and cash equivalents and debt securities available-for-sale	657,696	381,858
Gross Unrealized Gain	1,433	235
Gross Unrealized Loss	(39)	(317)
Estimated Fair Value, Cash, Cash Equivalents and Debt Securities	659,090	381,776
Money market funds
Debt Securities, Available-for-sale [Line Items]
Cash and cash equivalents	10,734	25,796
Estimated Fair Value, Cash and Cash Equivalents	10,734	25,796
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost, debt securities, available-for-sale	16,679	42,273
Gross Unrealized Gain	11	0
Gross Unrealized Loss	0	(71)
Estimated Fair Value, Debt Securities	16,690	42,202
U.S. government debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost, debt securities, available-for-sale	630,283	308,775
Gross Unrealized Gain	1,422	235
Gross Unrealized Loss	(39)	(236)
Estimated Fair Value, Debt Securities	$ 631,666	308,774
U.S. government agency bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost, debt securities, available-for-sale		5,014
Gross Unrealized Gain		0
Gross Unrealized Loss		(10)
Estimated Fair Value, Debt Securities		$ 5,004

Cash, Cash Equivalents and Debt Securities, Fair Value

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Debt Securities, Available-for-sale, and Cash and Cash Equivalents 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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