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Pension plans and other postretirement benefits (Tables) | 9 Months Ended
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Sep. 30, 2025
Retirement Benefits [Abstract] |
Net Periodic Defined Benefit Cost for Defined Benefit Plans | The Company provides defined pension and other postretirement benefits (including health care and life insurance benefits) to qualified retired employees. Net periodic benefit for defined benefit plans consisted of the following:
| Pension Benefits |  | Other Postretirement Benefits
| Three Months Ended September 30,
(Dollars in millions) | 2025 |  | 2024 |  | 2025 |  | 2024
Service cost | $ | 2 |  |  | $ | 3 |  |  | $ | — |  |  | $ | — |
Interest cost on projected benefit obligation | 27 |  |  | 28 |  |  | 1 |  |  | 1 |
Expected return on plan assets | (46) |  |  | (50) |  |  | — |  |  | — |
Amortization of prior service credit | — |  |  | — |  |  | (1) |  |  | (1) |
Amortization of net actuarial gain | (1) |  |  | — |  |  | — |  |  | (1) |
Net periodic benefit | $ | (18) |  |  | $ | (19) |  |  | $ | — |  |  | $ | (1) |
| Pension Benefits |  | Other Postretirement Benefits
| Nine Months Ended September 30,
(Dollars in millions) | 2025 |  | 2024 |  | 2025 |  | 2024
Service cost | $ | 6 |  |  | $ | 8 |  |  | $ | 1 |  |  | $ | 1 |
Interest cost on projected benefit obligation | 81 |  |  | 86 |  |  | 2 |  |  | 2 |
Expected return on plan assets | (139) |  |  | (151) |  |  | — |  |  | — |
Amortization of prior service credit | — |  |  | — |  |  | (2) |  |  | (2) |
Amortization of net actuarial gain | (2) |  |  | (1) |  |  | (2) |  |  | (2) |
Net periodic benefit | $ | (54) |  |  | $ | (58) |  |  | $ | (1) |  |  | $ | (1) |

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- Definition Tabular disclosure of one or more of the entity's defined benefit pension plans or one or more other defined benefit postretirement plans, separately for pension plans and other postretirement benefit plans including the entity's schedule of fair value of plan assets for defined benefit or other postretirement plans. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 715 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 5 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-5 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 50 -Paragraph 1 -SubTopic 20 -Topic 715 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-1 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 55 -Paragraph 17 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480482/715-20-55-17 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 3 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-3 Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 4 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-4 Reference 6: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480506/715-20-50-2 + Details
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