Fair Value Measurements. Cash Equivalents and Marketable Securities - Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Cash and cash equivalents	$ 174,653	$ 140,544
Amortized cost, cash and cash equivalents and debt securities available-for-sale	676,042	381,858
Gross Unrealized Gain	1,260	235
Gross Unrealized Loss	(4)	(317)
Cash, cash equivalents and debt securities, fair value	677,298	381,776
Money market funds
Debt Securities, Available-for-sale [Line Items]
Cash and cash equivalents	24,016	25,796
Estimated fair value, cash and cash equivalents	24,016	25,796
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost, debt securities, available-for-sale	32,919	42,273
Gross Unrealized Gain	69	0
Gross Unrealized Loss	0	(71)
Estimated fair value, debt securities	32,988	42,202
U.S. government debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized cost, debt securities, available-for-sale	614,082	308,775
Gross Unrealized Gain	1,189	235
Gross Unrealized Loss	(4)	(236)
Estimated fair value, debt securities	615,267	308,774
U.S. government agency bonds
Debt Securities, Available-for-sale [Line Items]
Amortized cost, debt securities, available-for-sale	5,025	5,014
Gross Unrealized Gain	2	0
Gross Unrealized Loss	0	(10)
Estimated fair value, debt securities	$ 5,027	$ 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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