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Fair Value (Tables) | 3 Months Ended
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Mar. 31, 2025
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 Condensed Consolidated Balance Sheets (unaudited) on a recurring basis and their level within the fair value hierarchy as of March 31, 2025 and December 31, 2024:
Recurring Fair Value Measurements March 31, 2025 (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 March 31, 2025
Assets |  |  |  |  |  |  |
U.S. Treasury debt securities(1) | $ | 41.5 |  |  | $ | — |  |  | $ | — |  |  | $ | 41.5 |
Risk management assets | — |  |  | 51.7 |  |  | — |  |  | 51.7 |
Available-for-sale debt securities | — |  |  | 81.2 |  |  | — |  |  | 81.2 |
Total | $ | 41.5 |  |  | $ | 132.9 |  |  | $ | — |  |  | $ | 174.4 |
Liabilities |  |  |  |  |  |  |
Risk management liabilities | $ | — |  |  | $ | 3.6 |  |  | $ | — |  |  | $ | 3.6 |
Total | $ | — |  |  | $ | 3.6 |  |  | $ | — |  |  | $ | 3.6 |
(1) Treasury bills are presented in "Cash and cash equivalents" and "Restricted cash" on the Consolidated Balance Sheets.
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 |
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 March 31, 2025 and December 31, 2024 were:
March 31, 2025 (in millions) | Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses(1) |  | Allowance for Credit Losses |  | Fair Value
Available-for-sale debt securities |  |  |  |  |  |  |  |  |
Corporate/Other debt securities | $ | 85.5 |  |  | $ | 0.6 |  |  | $ | (4.7) |  |  | $ | (0.2) |  |  | $ | 81.2 |
Total | $ | 85.5 |  |  | $ | 0.6 |  |  | $ | (4.7) |  |  | $ | (0.2) |  |  | $ | 81.2 |
December 31, 2024 (in millions) | Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses(2) |  | Allowance for Credit Losses |  | Fair 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 |
(1)Fair value of Corporate/Other debt securities in an unrealized loss position without an allowance for credit losses is $63.7 million at March 31, 2025. (2)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.
Carrying Amount And Estimated Fair Values Of Financial Instruments | The carrying amount and estimated fair values of these financial instruments were as follows:
(in millions) | Carrying Amount as of March 31, 2025 |  | Estimated Fair Value as of March 31, 2025 |  | Carrying Amount as of Dec. 31, 2024 |  | Estimated Fair Value as of Dec. 31, 2024
Long-term debt (including current portion) | $ | 14,114.1 |  |  | $ | 13,292.4 |  |  | $ | 13,355.7 |  |  | $ | 12,505.2 |

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- Definition Tabular disclosure of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 9 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-9 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-3 Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-2 Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-2 Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-2 Reference 6: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (aa) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-2 Reference 7: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (aaa) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-2 Reference 8: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-2 Reference 9: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-3 Reference 10: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-3 Reference 11: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-3 Reference 12: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 320 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 3 -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481800/320-10-50-3 + Details
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- Definition Tabular disclosure of assets and liabilities, including [financial] instruments measured at fair value that are classified in stockholders' equity, if any, that are measured at fair value on a recurring basis. The disclosures contemplated herein include the fair value measurements at the reporting date by the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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