Balance Sheet Details - Short-term Marketable Securities, Available-for-Sale (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Financial Statement Details [Line Items]
Amortized Cost	$ 1,890.0	$ 1,086.7
Gross Unrealized Gains	0.2	0.5
Gross Unrealized Losses	(0.1)	(0.1)
Estimated Market Value	1,890.1	1,087.1
U.S. government agencies
Financial Statement Details [Line Items]
Amortized Cost	1,570.4	675.6
Gross Unrealized Gains	0.1	0.4
Gross Unrealized Losses	(0.1)	0.0
Estimated Market Value	1,570.4	676.0
Commercial paper
Financial Statement Details [Line Items]
Amortized Cost	258.7	248.1
Gross Unrealized Gains	0.1	0.1
Gross Unrealized Losses	0.0	0.0
Estimated Market Value	258.8	248.2
Corporate debt
Financial Statement Details [Line Items]
Amortized Cost	60.9	163.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	(0.1)
Estimated Market Value	$ 60.9	$ 162.9

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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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