Fair Value Measurements - Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, cost	$ 1,961,972	$ 1,059,525
Gross unrealized gains	1,943	1
Gross unrealized losses	(459)	(4,770)
Available-for-sale securities	1,963,456	1,054,756
Short-term marketable securities	1,143,006	855,499
Long-term marketable securities	820,450	199,257
Certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, cost	150,000	40,000
Gross unrealized gains	0	0
Gross unrealized losses	0	(7)
Available-for-sale securities	150,000	39,993
Short-term marketable securities	150,000	39,993
Long-term marketable securities	0	0
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, cost	73,829	282,996
Gross unrealized gains	23	0
Gross unrealized losses	(7)	(50)
Available-for-sale securities	73,845	282,946
Short-term marketable securities	73,845	282,946
Long-term marketable securities	0	0
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, cost	1,368,668	685,653
Gross unrealized gains	1,840	1
Gross unrealized losses	(378)	(4,309)
Available-for-sale securities	1,370,130	681,345
Short-term marketable securities	753,538	482,088
Long-term marketable securities	616,592	199,257
U.S. government agency obligations
Debt Securities, Available-for-sale [Line Items]
Available-for-sale securities, cost	369,475	50,876
Gross unrealized gains	80	0
Gross unrealized losses	(74)	(404)
Available-for-sale securities	369,481	50,472
Short-term marketable securities	165,623	50,472
Long-term marketable securities	$ 203,858	$ 0

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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