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Fair Value (Tables)
 12 Months Ended
Dec. 31, 2024
Fair Value Disclosures [Abstract]  
Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis
The following tables present financial assets and liabilities measured and recorded at fair value on our Consolidated Balance Sheets on a recurring basis and their level within the fair value hierarchy as of December 31, 2024 and December 31, 2023:
 
Recurring Fair Value Measurements
December 31, 2024 (in millions)
Quoted Prices
in Active Markets
for Identical Assets
(Level 1)		Significant Other
Observable Inputs
(Level 2)		Significant
Unobservable
Inputs
(Level 3)
Balance as of
December 31, 2024
Assets
U.S. Treasury debt securities(1)
$80.1 $— $— $80.1 
Risk management assets— 27.0 — 27.0 
Available-for-sale debt securities— 86.7 — 86.7 
Total$80.1 $113.7 $— $193.8 
Liabilities
Risk management liabilities$— $3.5 $— $3.5 
Total$— $3.5 $— $3.5 
(1) Treasury bills are presented in "Cash and cash equivalents" and "Restricted cash" on the Consolidated Balance Sheets.
 
Recurring Fair Value Measurements
December 31, 2023 (in millions)
Quoted Prices
in Active Markets
for Identical Assets
(Level 1)		Significant Other
Observable Inputs
(Level 2)		Significant
Unobservable
Inputs
(Level 3)
Balance as of
December 31, 2023
Assets
Risk management assets$— $23.3 $— $23.3 
Available-for-sale debt securities— 159.1 — 159.1 
Total$— $182.4 $— $182.4 
Liabilities
Risk management liabilities$— $9.4 $— $9.4 
Total$— $9.4 $— $9.4 
Debt Securities, Available-for-sale
The amortized cost, gross unrealized gains and losses, allowance for credit losses, and fair value of available-for-sale securities at December 31, 2024 and 2023 were: 
December 31, 2024 (in millions)
Amortized
Cost		Gross
Unrealized
Gains
Gross
Unrealized
Losses(1)
Allowance for Credit LossesFair Value
Available-for-sale debt securities
Corporate/Other debt securities$91.9 $0.5 $(5.6)$(0.1)$86.7 
Total$91.9 $0.5 $(5.6)$(0.1)$86.7 
December 31, 2023 (in millions)
Amortized
Cost		Gross
Unrealized
Gains
Gross
Unrealized
Losses(2)
Allowance for Credit LossesFair Value
Available-for-sale debt securities
U.S. Treasury debt securities$63.8 $— $(3.2)$— $60.6 
Corporate/Other debt securities105.2 0.8 (6.9)(0.6)98.5 
Total$169.0 $0.8 $(10.1)$(0.6)$159.1 
(1) Fair value of Corporate/Other debt securities in an unrealized loss position without an allowance for credit losses is $70.1 million at December 31, 2024.
(2) Fair value of U.S. Treasury debt securities and Corporate/Other debt securities in an unrealized loss position without an allowance for credit losses is $58.7 million and $74.8 million, respectively, at December 31, 2023.
Carrying Amount And Estimated Fair Values Of Financial Instruments
The carrying amount and estimated fair values of these financial instruments were as follows: 
At December 31, (in millions)
Carrying
Amount
2024
Estimated
Fair Value
2024
Carrying
Amount
2023
Estimated
Fair Value
2023
Long-term debt (including current portion)$13,355.7 $12,505.2 $11,079.3 $10,370.9