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Fair Value Measurements - Fair Value Hierarchy of Financial Assets (Details) - USD ($)
$ in Millions
Dec. 31, 2021
Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents $ 831.4 $ 641.5
Debt securities, available-for-sale 1,678.6 1,890.1
Other assets 7.0 3.4
Total assets measured at fair value on a recurring basis 2,517.0 2,535.0
Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 791.4 491.5
Debt securities, available-for-sale 0.0 0.0
Other assets 7.0 3.4
Total assets measured at fair value on a recurring basis 798.4 494.9
Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents 40.0 150.0
Debt securities, available-for-sale 1,678.6 1,890.1
Other assets 0.0 0.0
Total assets measured at fair value on a recurring basis 1,718.6 2,040.1
Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
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 [Line Items]    
Debt securities, available-for-sale 1,210.6 1,570.4
U.S. government agencies | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
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 [Line Items]    
Debt securities, available-for-sale 1,210.6 1,570.4
U.S. government agencies | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities, available-for-sale 0.0 0.0
Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities, available-for-sale 189.7 258.8
Commercial paper | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities, available-for-sale 0.0 0.0
Commercial paper | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities, available-for-sale 189.7 258.8
Commercial paper | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities, available-for-sale 0.0 0.0
Corporate debt    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities, available-for-sale 224.3 60.9
Corporate debt | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities, available-for-sale 0.0 0.0
Corporate debt | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities, available-for-sale 224.3 60.9
Corporate debt | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities, available-for-sale 0.0 $ 0.0
Supranationals    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities, available-for-sale 54.0  
Supranationals | Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities, available-for-sale 0.0  
Supranationals | Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities, available-for-sale 54.0  
Supranationals | Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Debt securities, available-for-sale $ 0.0