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Debt Securities - Amortized Cost, Gross Unrealized Gains and Losses, Allowance for Credit Losses and Fair Value of Debt Securities Available-For-Sale (Details) - USD ($)
$ in Millions
Sep. 30, 2023
Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, current $ 804.8 $ 735.8
Unrealized gain, current 0.0 0.0
Unrealized loss, current (3.4) (9.4)
Fair value, current 801.4 726.4
Amortized cost, noncurrent 458.6 304.5
Unrealized gain, noncurrent 0.1 0.2
Unrealized loss, noncurrent (4.0) (5.3)
Fair value, noncurrent 454.7 299.4
Commercial paper    
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, current 137.4 156.2
Unrealized gain, current 0.0 0.0
Unrealized loss, current 0.0 (0.2)
Fair value, current 137.4 156.0
Corporate debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, current 306.6 296.2
Unrealized gain, current 0.0 0.0
Unrealized loss, current (1.9) (3.2)
Fair value, current 304.7 293.0
Amortized cost, noncurrent 341.3 259.5
Unrealized gain, noncurrent 0.1 0.2
Unrealized loss, noncurrent (3.1) (4.3)
Fair value, noncurrent 338.3 255.4
Securities of government-sponsored entities    
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, current 360.8 283.4
Unrealized gain, current 0.0 0.0
Unrealized loss, current (1.5) (6.0)
Fair value, current 359.3 277.4
Amortized cost, noncurrent 117.3 45.0
Unrealized gain, noncurrent 0.0 0.0
Unrealized loss, noncurrent (0.9) (1.0)
Fair value, noncurrent $ 116.4 $ 44.0