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Significant Segment Financial Data (Detail) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 | Mar. 31, 2017 | Dec. 31, 2017
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Other Operating Revenue - Non-Regulated | $ 1.3 |  |
Revenues | 145.8 | $ 126.0 |
Segment Profit | 15.6 | 12.4 |
Identifiable Segment Assets | 1,244.3 | 1,134.4 | $ 1,241.9
Capital Expenditures | 10.1 | 17.5 |
Billed and Unbilled Revenue |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Revenues | 153.0 |  |
Rate Adjustment Mechanism Revenue |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Revenues | (8.5) |  |
Gas Segment |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Revenues | 87.0 | 74.8 |
Segment Profit | 12.6 | 9.9 |
Identifiable Segment Assets | 712.6 | 641.8 | 714.3
Capital Expenditures | 3.6 | 5.7 |
Gas Segment | Billed and Unbilled Revenue |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Revenues | 91.4 |  |
Gas Segment | Rate Adjustment Mechanism Revenue |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Revenues | (4.4) |  |
Electric |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Revenues | 57.5 | 49.5 |
Segment Profit | 3.0 | 2.1 |
Identifiable Segment Assets | 481.8 | 442.8 | 476.9
Capital Expenditures | 6.0 | 8.3 |
Electric | Billed and Unbilled Revenue |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Revenues | 61.6 |  |
Electric | Rate Adjustment Mechanism Revenue |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Revenues | (4.1) |  |
All Other Segments |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Segment Profit | (0.4) |  |
Identifiable Segment Assets | 42.8 | 42.8 | 44.0
Capital Expenditures | 0.5 | 3.5 |
Unregulated Operation |  |  |
Segment Reporting, Reconciling Item for Operating Profit (Loss) from Segment to Consolidated [Line Items] |  |  |
Other Operating Revenue - Non-Regulated | 1.3 |  |
Revenues | 1.3 | 1.7 |
Segment Profit | 0.4 | 0.4 |
Identifiable Segment Assets | $ 7.1 | $ 7.0 | $ 6.7

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