Marketable Securities (Available-for-sale Investments by Classification in Balance Sheet) (Details) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 0.0	$ 369.0
Cash and Cash Equivalents
Debt Securities, Available-for-sale [Line Items]
Fair Value	0.0	328.1
Marketable securities
Debt Securities, Available-for-sale [Line Items]
Fair Value	$ 0.0	40.9
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		246.9
Gross Unrealized Holding Gains		0.0
Gross Unrealized Holding Losses		0.0
Fair Value		246.9
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		24.3
Gross Unrealized Holding Gains		0.0
Gross Unrealized Holding Losses		0.0
Fair Value		24.3
U.S.
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		70.4
Gross Unrealized Holding Gains		0.0
Gross Unrealized Holding Losses		0.0
Fair Value		70.4
Bank certificates of deposit
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		27.4
Gross Unrealized Holding Gains		0.0
Gross Unrealized Holding Losses		0.0
Fair Value		27.4
Total available-for-sale debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		369.0
Gross Unrealized Holding Gains		0.0
Gross Unrealized Holding Losses		0.0
Fair Value		$ 369.0

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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