Fair Value Measurements - Summary (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Cash equivalents	$ 463.7	$ 255.0
Debt securities, available for sale	925.9	1,087.1
Other assets	1.8	0.7
Total assets measured at fair value on a recurring basis	1,391.4	1,342.8
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Cash equivalents	363.7	110.1
Debt securities, available for sale	0.0	0.0
Other assets	1.8	0.7
Total assets measured at fair value on a recurring basis	365.5	110.8
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Cash equivalents	100.0	144.9
Debt securities, available for sale	925.9	1,087.1
Other assets	0.0	0.0
Total assets measured at fair value on a recurring basis	1,025.9	1,232.0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Cash equivalents	0.0	0.0
Debt securities, available for sale	0.0	0.0
Other assets	0.0	0.0
Total assets measured at fair value on a recurring basis	0.0	0.0
U.S. government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Debt securities, available for sale	623.9	676.0
U.S. government agencies \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Debt securities, available for sale	0.0	0.0
U.S. government agencies \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Debt securities, available for sale	623.9	676.0
U.S. government agencies \| Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Debt securities, available for sale	0.0	0.0
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Debt securities, available for sale	189.1	248.2
Commercial paper \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Debt securities, available for sale	0.0	0.0
Commercial paper \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Debt securities, available for sale	189.1	248.2
Commercial paper \| Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Debt securities, available for sale	0.0	0.0
Corporate debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Debt securities, available for sale	112.9	162.9
Corporate debt \| Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Debt securities, available for sale	0.0	0.0
Corporate debt \| Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Debt securities, available for sale	112.9	162.9
Corporate debt \| Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Debt securities, available for sale	$ 0.0	$ 0.0

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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+ References

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Fair value portion of other assets.

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