Fair Value Measurements - Assets Measured at Fair Value on a Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value Measurements, Financial Assets
Cash and cash equivalents	$ 275,948	$ 189,052
Total amortized cost	1,504,281	1,594,334
Marketable securities, accumulated gross unrealized gain, before tax	4,483	4,757
Marketable securities, gross unrealized losses less than 12 months	(10,895)	(58)
Marketable securities, aggregate fair value	1,497,869	1,599,033
Strategic investments, amortized cost	5,750	5,500
Strategic investments, gross unrealized gains	1,944
Strategic investments	7,694	5,500
Total financial assets, amortized cost or carrying value	1,785,979	1,788,886
Total gross unrealized gains	6,427	4,757
Total financial assets	1,781,511	1,793,585
Note receivable	7,700
U.S. Treasury securities
Fair Value Measurements, Financial Assets
Total amortized cost	193,234	215,847
Marketable securities, accumulated gross unrealized gain, before tax	1,927	241
Marketable securities, gross unrealized losses less than 12 months	0	(3)
Marketable securities, aggregate fair value	195,161	216,085
Corporate debt securities and commercial paper
Fair Value Measurements, Financial Assets
Total amortized cost	1,311,047	1,378,487
Marketable securities, accumulated gross unrealized gain, before tax	2,556	4,516
Marketable securities, gross unrealized losses less than 12 months	(10,895)	(55)
Marketable securities, aggregate fair value	1,302,708	1,382,948
Level 1
Fair Value Measurements, Financial Assets
Cash and cash equivalents	275,948	153,252
Marketable securities, aggregate fair value	206,161	221,085
Total financial assets	482,109	374,337
Level 1 \| U.S. Treasury securities
Fair Value Measurements, Financial Assets
Marketable securities, aggregate fair value	195,161	216,085
Level 1 \| Corporate debt securities and commercial paper
Fair Value Measurements, Financial Assets
Marketable securities, aggregate fair value	11,000	5,000
Level 2
Fair Value Measurements, Financial Assets
Cash and cash equivalents	0	35,800
Marketable securities, aggregate fair value	1,291,708	1,377,948
Total financial assets	1,291,708	1,413,748
Level 2 \| U.S. Treasury securities
Fair Value Measurements, Financial Assets
Marketable securities, aggregate fair value	0	0
Level 2 \| Corporate debt securities and commercial paper
Fair Value Measurements, Financial Assets
Marketable securities, aggregate fair value	1,291,708	1,377,948
Level 3
Fair Value Measurements, Financial Assets
Strategic investments	7,694	5,500
Total financial assets	7,694	5,500
Carrying Value \| Money market funds
Fair Value Measurements, Financial Assets
Cash and cash equivalents	275,948	153,252
Carrying Value \| Reverse repurchase agreements
Fair Value Measurements, Financial Assets
Cash and cash equivalents		35,800
Aggregate Fair Value \| Money market funds
Fair Value Measurements, Financial Assets
Cash and cash equivalents	275,948	153,252
Aggregate Fair Value \| Reverse repurchase agreements
Fair Value Measurements, Financial Assets
Cash and cash equivalents		35,800
Aggregate Fair Value \| Level 1 \| Money market funds
Fair Value Measurements, Financial Assets
Cash and cash equivalents	$ 275,948	153,252
Aggregate Fair Value \| Level 2 \| Reverse repurchase agreements
Fair Value Measurements, Financial Assets
Cash and cash equivalents		$ 35,800

Debt Securities, Available-For-Sale, Privately Held

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Debt Securities, Available-For-Sale, Privately Held, Amortized Cost

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Debt Securities, Available-For-Sale, Privately Held, Gross Unrealized Gains

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Financial Assets, Accumulated Gross Unrealized Gain, Before Tax

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The aggregate of carrying value for assets with carrying value that approximates fair value and amortized cost basis for assets carried at fair value.

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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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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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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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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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Fair value portion of an agreement for an unconditional promise by the maker to pay the holder a definite sum of money at a future date.

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