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Available-for-Sale Debt Securities
 9 Months Ended
Sep. 30, 2024
Investments, Debt and Equity Securities [Abstract]  
Available-for-Sale Debt Securities Available-for-Sale Debt Securities
The following table presents a summary of available-for-sale debt securities, aggregated by major security type and contractual maturity.
September 30,
2024		December 31,
2023
(in millions)Contractual
Maturity		Amortized
Cost		Unrealized
Gain		Unrealized
Loss		Fair
Value		Amortized
Cost		Unrealized
Gain		Unrealized
Loss		Fair
Value
Commercial paper0 to 1 years$37.3 $— $— $37.3 $53.5 $— $— $53.5 
Corporate debt securities0 to 1 years531.4 1.0 (0.1)532.3 382.1 0.1 (1.0)381.2 
Securities of government-sponsored entities0 to 1 years308.7 0.6 — 309.3 346.1 0.2 (0.5)345.8 
$877.4 $1.6 $(0.1)$878.9 $781.7 $0.3 $(1.5)$780.5 
Corporate debt securities1 to 3 years$488.9 $6.2 $(0.1)$495.0 $483.5 $2.9 $(0.4)$486.0 
Securities of government-sponsored entities1 to 3 years148.2 0.8 (0.1)148.9 201.1 0.5 (0.1)201.5 
$637.1 $7.0 $(0.2)$643.9 $684.6 $3.4 $(0.5)$687.5 
Unrealized losses on available-for-sale debt securities were primarily due to changes in interest rates. These investments are of high credit quality, and we do not intend to sell these investments and it is not more likely than not that we will be required to sell these investments before recovery of their amortized cost basis. No allowance for credit losses was recognized as of September 30, 2024 or December 31, 2023.
The following table presents available-for-sale debt securities that were in an unrealized loss position as of September 30, 2024, aggregated by major security type and length of time in a continuous loss position.
Less Than 12 Months12 Months or LongerTotal
(in millions)Fair
Value		Unrealized
Loss		Fair
Value		Unrealized
Loss		Fair
Value		Unrealized
Loss
Corporate debt securities$125.9 $(0.1)$49.1 $(0.1)$175.0 $(0.2)
Securities of government-sponsored entities$84.8 $(0.1)$— $— $84.8 $(0.1)
The following table presents available-for-sale debt securities that were in an unrealized loss position as of December 31, 2023, aggregated by major security type and length of time in a continuous loss position.
Less Than 12 Months12 Months or LongerTotal
(in millions)Fair
Value		Unrealized
Loss		Fair
Value		Unrealized
Loss		Fair
Value		Unrealized
Loss
Corporate debt securities$265.1 $(0.4)$183.8 $(1.0)$448.9 $(1.4)
Securities of government-sponsored entities$214.6 $(0.2)$16.7 $(0.4)$231.3 $(0.6)
Accrued interest receivables on available-for-sale debt securities totaled $13.5 million and $11.2 million, respectively, as of September 30, 2024 and December 31, 2023. We do not measure an allowance for credit losses for accrued interest receivables. For the purposes of identifying and measuring an impairment, accrued interest is excluded from both the fair value and amortized cost basis of the debt security. Uncollectible accrued interest receivables associated with an impaired debt security are reversed against interest income upon identification of the impairment. No accrued interest receivables were written off during the first nine months of 2024 or 2023.