Fair Value Measurements. Cash Equivalents and Marketable Securities - Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Cash and cash equivalents	$ 152,239	$ 143,228
Amortized cost, cash and cash equivalents and debt securities available-for-sale	633,691	657,696
Gross Unrealized Gain	7,961	1,433
Gross Unrealized Loss	0	(39)
Cash, cash equivalents and debt securities, fair value	641,652	659,090
Money market funds
Debt Securities, Available-for-sale [Line Items]
Cash and cash equivalents	35,592	10,734
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Estimated fair value, cash and cash equivalents	35,592	10,734
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost, debt securities, available-for-sale	4,035	16,679
Gross Unrealized Gain	0	11
Gross Unrealized Loss	0	0
Estimated fair value, debt securities	4,035	16,690
U.S. government debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost, debt securities, available-for-sale	594,064	630,283
Gross Unrealized Gain	7,961	1,422
Gross Unrealized Loss	0	(39)
Estimated fair value, debt securities	$ 602,025	$ 631,666

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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