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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
Mar. 31, 2024
Dec. 31, 2023
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, current $ 815.7 $ 781.7
Unrealized gain, current 0.1 0.3
Unrealized loss, current (1.5) (1.5)
Fair value, current 814.3 780.5
Amortized cost, noncurrent 701.4 684.6
Unrealized gain, noncurrent 0.9 3.4
Unrealized loss, noncurrent (1.9) (0.5)
Fair value, noncurrent 700.4 687.5
Commercial paper    
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, current 58.0 53.5
Unrealized gain, current 0.0 0.0
Unrealized loss, current 0.0 0.0
Fair value, current 58.0 53.5
Corporate debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, current 419.4 382.1
Unrealized gain, current 0.1 0.1
Unrealized loss, current (0.9) (1.0)
Fair value, current 418.6 381.2
Amortized cost, noncurrent 530.0 483.5
Unrealized gain, noncurrent 0.9 2.9
Unrealized loss, noncurrent (1.3) (0.4)
Fair value, noncurrent 529.6 486.0
Securities of government-sponsored entities    
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, current 338.3 346.1
Unrealized gain, current 0.0 0.2
Unrealized loss, current (0.6) (0.5)
Fair value, current 337.7 345.8
Amortized cost, noncurrent 171.4 201.1
Unrealized gain, noncurrent 0.0 0.5
Unrealized loss, noncurrent (0.6) (0.1)
Fair value, noncurrent $ 170.8 $ 201.5