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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
Dec. 31, 2023
Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, current $ 781.7 $ 735.8
Unrealized gain, current 0.3 0.0
Unrealized loss, current (1.5) (9.4)
Fair value, current 780.5 726.4
Amortized cost, noncurrent 684.6 304.5
Unrealized gain, noncurrent 3.4 0.2
Unrealized loss, noncurrent (0.5) (5.3)
Fair value, noncurrent 687.5 299.4
Commercial paper    
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, current 53.5 156.2
Unrealized gain, current 0.0 0.0
Unrealized loss, current 0.0 (0.2)
Fair value, current 53.5 156.0
Corporate debt securities    
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, current 382.1 296.2
Unrealized gain, current 0.1 0.0
Unrealized loss, current (1.0) (3.2)
Fair value, current 381.2 293.0
Amortized cost, noncurrent 483.5 259.5
Unrealized gain, noncurrent 2.9 0.2
Unrealized loss, noncurrent (0.4) (4.3)
Fair value, noncurrent 486.0 255.4
Securities of government-sponsored entities    
Debt Securities, Available-for-sale [Line Items]    
Amortized cost, current 346.1 283.4
Unrealized gain, current 0.2 0.0
Unrealized loss, current (0.5) (6.0)
Fair value, current 345.8 277.4
Amortized cost, noncurrent 201.1 45.0
Unrealized gain, noncurrent 0.5 0.0
Unrealized loss, noncurrent (0.1) (1.0)
Fair value, noncurrent $ 201.5 $ 44.0