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Significant Segment Financial Data (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Other Operating Revenue – Non-Regulated |  |  | $ 0.9
Total Operating Revenues | $ 473.3 | $ 418.6 | 438.2
Interest Income | 1.6 | 2.6 | 2.9
Interest Expense | 27.2 | 26.4 | 26.6
Depreciation & Amortization Expense | 59.5 | 54.5 | 52.0
Income Tax Expense (Benefit) | 11.5 | 10.2 | 13.8
Segment Profit (Loss) | 36.1 | 32.2 | 44.2
Segment Assets | 1,540.3 | 1,477.9 | 1,370.8
Capital Expenditures | 115.0 | 122.6 | 119.2
Billed and Unbilled Revenue |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Total Operating Revenues | 466.1 | 411.9 | 435.2
Rate Adjustment Mechanism Revenue |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Total Operating Revenues | 7.2 | 6.7 | 2.1
Electric |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Total Operating Revenues | 248.5 | 227.2 | 233.9
Interest Income | 0.8 | 1.1 | 0.9
Interest Expense | 9.0 | 8.7 | 9.4
Depreciation & Amortization Expense | 25.9 | 23.8 | 22.6
Income Tax Expense (Benefit) | 4.5 | 4.7 | 4.2
Segment Profit (Loss) | 14.0 | 12.9 | 11.5
Segment Assets | 584.0 | 571.8 | 529.3
Capital Expenditures | 38.1 | 45.5 | 39.6
Electric | Billed and Unbilled Revenue |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Total Operating Revenues | 248.5 | 226.7 | 223.1
Electric | Rate Adjustment Mechanism Revenue |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Total Operating Revenues | 0.0 | 0.5 | 10.8
Gas Segment |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Total Operating Revenues | 224.8 | 191.4 | 203.4
Interest Income | 0.5 | 1.1 | 1.2
Interest Expense | 15.3 | 14.2 | 14.4
Depreciation & Amortization Expense | 32.6 | 29.8 | 28.5
Income Tax Expense (Benefit) | 7.7 | 7.3 | 7.2
Segment Profit (Loss) | 23.2 | 19.3 | 19.1
Segment Assets | 935.9 | 886.3 | 823.3
Capital Expenditures | 75.8 | 71.1 | 74.0
Gas Segment | Billed and Unbilled Revenue |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Total Operating Revenues | 217.6 | 185.2 | 212.1
Gas Segment | Rate Adjustment Mechanism Revenue |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Total Operating Revenues | 7.2 | 6.2 |
All Other Segments |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Interest Income | 0.3 | 0.4 | 0.6
Interest Expense | 2.9 | 3.5 | 2.8
Depreciation & Amortization Expense | 1.0 | 0.9 | 0.9
Income Tax Expense (Benefit) | (0.6) | (1.8) | (1.4)
Segment Profit (Loss) | (1.2) |  | 3.4
Segment Assets | 20.4 | 19.8 | 17.9
Capital Expenditures | 1.1 | $ 6.0 | 5.6
Unregulated Operation |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Other Operating Revenue – Non-Regulated |  |  | 0.9
Total Operating Revenues |  |  | 0.9
Interest Income |  |  | 0.2
Income Tax Expense (Benefit) | (0.1) |  | 3.8
Segment Profit (Loss) | $ 0.1 |  | 10.2
Segment Assets |  |  | $ 0.3

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