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Fair Value Measurements - Summary (Details) - USD ($) $ in Millions | Sep. 30, 2021 | Dec. 31, 2020
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Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Cash equivalents | $ 1,032.8 | $ 641.5
Debt securities, available for sale: | 1,253.6 | 1,890.1
Other assets | 6.3 | 3.4
Total assets measured at fair value on a recurring basis | 2,292.7 | 2,535.0
U.S. government agencies |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available for sale: | 898.8 | 1,570.4
Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available for sale: | 254.8 | 258.8
Corporate debt |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available for sale: | 100.0 | 60.9
Level 1 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Cash equivalents | 782.8 | 491.5
Debt securities, available for sale: | 0.0 | 0.0
Other assets | 6.3 | 3.4
Total assets measured at fair value on a recurring basis | 789.1 | 494.9
Level 1 | U.S. government agencies |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available for sale: | 0.0 | 0.0
Level 1 | Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available for sale: | 0.0 | 0.0
Level 1 | Corporate debt |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available for sale: | 0.0 | 0.0
Level 2 |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Cash equivalents | 250.0 | 150.0
Debt securities, available for sale: | 1,253.6 | 1,890.1
Other assets | 0.0 | 0.0
Total assets measured at fair value on a recurring basis | 1,503.6 | 2,040.1
Level 2 | U.S. government agencies |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available for sale: | 898.8 | 1,570.4
Level 2 | Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available for sale: | 254.8 | 258.8
Level 2 | Corporate debt |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available for sale: | 100.0 | 60.9
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
Level 3 | U.S. government agencies |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available for sale: | 0.0 | 0.0
Level 3 | Commercial paper |  |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract] |  |
Debt securities, available for sale: | 0.0 | 0.0
Level 3 | Corporate debt |  |
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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