Cash, Cash Equivalents and Short-Term Investments - Schedule of Components (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	$ 334.2	$ 551.3
Cash equivalents, unrealized loss	(0.1)
Total cash equivalents	334.1	551.3
Short-term investments, amortized cost	777.9	606.3
Short-term investments, unrealized gain	5.9	1.5
Total cash, cash equivalents, and short term investments, unrealized losses	0.4
Short-term investments, unrealized loss	(0.3)	(0.1)
Short-term investments, estimated fair value	783.5	607.7
Total cash, cash equivalents, and short term investments, before unrealized gains (losses) on investments	1,112.1	1,157.6
Total cash, cash equivalents, and short term investments	1,117.6	1,159.0
Corporate notes and obligations
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	358.3	285.5
Short-term investments, unrealized gain	3.3	1.2
Short-term investments, unrealized loss	(0.2)	(0.1)
Short-term investments, estimated fair value	361.4	286.6
US Treasury securities
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	234.1	171.0
Short-term investments, unrealized gain	1.7	0.3
Short-term investments, unrealized loss	0.0	0.0
Short-term investments, estimated fair value	235.8	171.3
Asset-backed securities
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	83.0	53.8
Short-term investments, unrealized gain	0.7	0.0
Short-term investments, unrealized loss	(0.1)	0.0
Short-term investments, estimated fair value	83.6	53.8
Commercial paper
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	40.0	24.2
Short-term investments, unrealized gain	0.0	0.0
Short-term investments, unrealized loss	0.0	0.0
Short-term investments, estimated fair value	40.0	24.2
US Agency securities
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	27.8	27.2
Short-term investments, unrealized gain	0.1	0.0
Short-term investments, unrealized loss	0.0	0.0
Short-term investments, estimated fair value	27.9	27.2
Certificates of deposit
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	26.0	38.2
Short-term investments, unrealized gain	0.0	0.0
Short-term investments, unrealized loss	0.0	0.0
Short-term investments, estimated fair value	26.0	38.2
Municipal securities
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	5.9	2.4
Short-term investments, unrealized gain	0.1	0.0
Short-term investments, unrealized loss	0.0	0.0
Short-term investments, estimated fair value	6.0	2.4
Foreign government obligations
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost	2.8
Short-term investments, unrealized gain	0.0
Short-term investments, unrealized loss	0.0
Short-term investments, estimated fair value	2.8
Supranational securities
Cash and Cash Equivalents [Line Items]
Short-term investments, amortized cost		4.0
Short-term investments, unrealized gain		0.0
Short-term investments, unrealized loss		0.0
Short-term investments, estimated fair value		4.0
Cash
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	88.5	105.0
Total cash equivalents	88.5	105.0
Money market funds
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	241.6	444.3
Cash equivalents, unrealized loss	(0.1)
Total cash equivalents	241.5	444.3
Municipal securities
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	1.8
Total cash equivalents	1.8
Commercial paper
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	1.2	2.0
Total cash equivalents	1.2	$ 2.0
Corporate notes and obligations
Cash and Cash Equivalents [Line Items]
Cash and cash equivalents	1.1
Total cash equivalents	$ 1.1

Cash, Cash Equivalents, And Short Term Investments, Accumulated Gross Unrealized Loss, Before Tax

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Cash, Cash Equivalents, And Short Term Investments, 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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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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Amount 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. Excludes cash and cash equivalents within disposal group and discontinued operation.

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