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Segment Information - Significant Segment Financial Data (Detail) - USD ($) $ in Millions | 3 Months Ended | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2024 | Jun. 30, 2023 | Jun. 30, 2024 | Jun. 30, 2023 | Dec. 31, 2023
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |  |
Total Operating Revenues | $ 95.7 | $ 103.4 | $ 274.4 | $ 323.6 |
Segment Profit (Loss) | 4.3 | 4.2 | 31.5 | 28.3 |
Capital Expenditures | 36.7 | 35.4 | 56.9 | 57.6 |
Segment Assets | 1,695.0 | 1,595.3 | 1,695.0 | 1,595.3 | $ 1,670.4
Billed and Unbilled Revenue |  |  |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |  |
Total Operating Revenues | 105.8 | 115.1 | 277.1 | 334.6 |
Rate Adjustment Mechanism Revenue |  |  |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |  |
Total Operating Revenues | (10.1) | (11.7) | (2.7) | (11.0) |
Gas Segment |  |  |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |  |
Total Operating Revenues | 39.3 | 38.9 | 144.4 | 150.9 |
Segment Profit (Loss) | 1.0 | 0.9 | 23.4 | 19.9 |
Capital Expenditures | 24.0 | 22.0 | 34.7 | 33.5 |
Segment Assets | 1,035.3 | 961.9 | 1,035.3 | 961.9 |
Gas Segment | Billed and Unbilled Revenue |  |  |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |  |
Total Operating Revenues | 47.0 | 45.3 | 147.3 | 167.2 |
Gas Segment | Rate Adjustment Mechanism Revenue |  |  |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |  |
Total Operating Revenues | (7.7) | (6.4) | (2.9) | (16.3) |
Electric |  |  |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |  |
Total Operating Revenues | 56.4 | 64.5 | 130.0 | 172.7 |
Segment Profit (Loss) | 3.4 | 3.6 | 8.3 | 8.9 |
Capital Expenditures | 11.7 | 13.4 | 20.4 | 24.0 |
Segment Assets | 632.9 | 610.5 | 632.9 | 610.5 |
Electric | Billed and Unbilled Revenue |  |  |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |  |
Total Operating Revenues | 58.8 | 69.8 | 129.8 | 167.4 |
Electric | Rate Adjustment Mechanism Revenue |  |  |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |  |
Total Operating Revenues | (2.4) | (5.3) | 0.2 | 5.3 |
All Other Segments |  |  |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |  |  |
Segment Profit (Loss) | (0.1) | (0.3) | (0.2) | (0.5) |
Capital Expenditures | 1.0 |  | 1.8 | 0.1 |
Segment Assets | $ 26.8 | $ 22.9 | $ 26.8 | $ 22.9 |

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