Fair Value Measurements - Fair Value Hierarchy for Financial Assets (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Cash equivalents	$ 581.7	$ 315.9
Debt securities, available-for-sale	2,181.5	2,157.8
Other assets	19.9	15.2
Total assets measured at fair value on a recurring basis	2,783.1	2,488.9
U.S. government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Debt securities, available-for-sale	1,233.3	1,612.5
Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Debt securities, available-for-sale	497.3	184.7
Corporate debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Debt securities, available-for-sale	450.9	360.6
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Cash equivalents	543.2	315.9
Debt securities, available-for-sale	0.0	0.0
Other assets	19.9	15.2
Total assets measured at fair value on a recurring basis	563.1	331.1
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	38.5	0.0
Debt securities, available-for-sale	2,181.5	2,157.8
Other assets	0.0	0.0
Total assets measured at fair value on a recurring basis	2,220.0	2,157.8
Level 2 \| U.S. government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Debt securities, available-for-sale	1,233.3	1,612.5
Level 2 \| Commercial paper
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Debt securities, available-for-sale	497.3	184.7
Level 2 \| Corporate debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Debt securities, available-for-sale	450.9	360.6
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