Marketable Securities and Equity Investments - Summary of Cash Equivalents and Marketable Securities (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2020	Dec. 31, 2019
Total marketable debt securities
Fair Value	$ 3,615,654	$ 1,243,469
Corporate equity securities
Amortized Cost	103,463	113,829
Gross Unrealized Gains	209,219	168,255
Gross Unrealized Losses	0	0
Fair Value	312,682	282,084
Amortized Cost	582,432	530,216
Gross Unrealized Gains	210,561	168,811
Gross Unrealized Losses	(22)	(55)
Fair Value	792,971	698,972
Cash equivalents
Total marketable debt securities
Amortized Cost	3,135,364	826,579
Gross Unrealized Gains	1	3
Gross Unrealized Losses	0	(1)
Fair Value	3,135,365	826,581
Cash equivalents \| Money market funds
Total marketable debt securities
Amortized Cost	3,130,364	791,039
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	3,130,364	791,039
Cash equivalents \| Corporate debt securities
Total marketable debt securities
Amortized Cost	0	6,070
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	0	6,070
Cash equivalents \| Commercial paper
Total marketable debt securities
Amortized Cost	5,000	29,470
Gross Unrealized Gains	1	3
Gross Unrealized Losses	0	(1)
Fair Value	5,001	29,472
Marketable securities
Total marketable debt securities
Amortized Cost	478,969	416,387
Gross Unrealized Gains	1,342	556
Gross Unrealized Losses	(22)	(55)
Fair Value	480,289	416,888
Marketable securities \| Government-sponsored enterprise securities
Total marketable debt securities
Amortized Cost	91,256	12,689
Gross Unrealized Gains	33	44
Gross Unrealized Losses	(6)	0
Fair Value	91,283	12,733
Marketable securities \| Corporate debt securities
Total marketable debt securities
Amortized Cost	275,554	301,458
Gross Unrealized Gains	1,067	391
Gross Unrealized Losses	(11)	(50)
Fair Value	276,610	301,799
Marketable securities \| Commercial paper
Total marketable debt securities
Amortized Cost	112,159	102,240
Gross Unrealized Gains	242	121
Gross Unrealized Losses	(5)	(5)
Fair Value	$ 112,396	$ 102,356

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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