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Fair Value Measurements - Summary (Details) - USD ($) $ in Millions | Jun. 30, 2020 | Dec. 31, 2019
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Cash equivalents | $ 402.6 | $ 255.0
Debt securities, available for sale | 1,978.8 | 1,087.1
Other assets | 2.4 | 0.7
Total assets measured at fair value on a recurring basis | 2,383.8 | 1,342.8
Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Cash equivalents | 402.6 | 110.1
Debt securities, available for sale | 0.0 | 0.0
Other assets | 2.4 | 0.7
Total assets measured at fair value on a recurring basis | 405.0 | 110.8
Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Cash equivalents | 0.0 | 144.9
Debt securities, available for sale | 1,978.8 | 1,087.1
Other assets | 0.0 | 0.0
Total assets measured at fair value on a recurring basis | 1,978.8 | 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 | 1,600.2 | 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 | 1,600.2 | 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 | 295.3 | 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 | 295.3 | 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 | 83.3 | 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 | 83.3 | 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

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