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Components of Gas and Electric Operating Revenue (Detail) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Operating Revenues [Line Items] |  |  |
Total Gas and Electric Operating Revenues | $ 439.4 | $ 400.2 | $ 377.3
Billed and Unbilled Revenue |  |  |
Operating Revenues [Line Items] |  |  |
Total Gas and Electric Operating Revenues | 439.4 | 389.1 | 353.7
Rate Adjustment Mechanism Revenue |  |  |
Operating Revenues [Line Items] |  |  |
Total Gas and Electric Operating Revenues |  | 11.1 | 23.6
Gas Segment |  |  |
Operating Revenues [Line Items] |  |  |
Total Gas and Electric Operating Revenues | 216.1 | 194.0 | 181.2
Gas Segment | Billed and Unbilled Revenue |  |  |
Operating Revenues [Line Items] |  |  |
Total Gas and Electric Operating Revenues | 210.7 | 187.5 | 165.4
Gas Segment | Rate Adjustment Mechanism Revenue |  |  |
Operating Revenues [Line Items] |  |  |
Total Gas and Electric Operating Revenues | 5.4 | 6.5 | 15.8
Electric |  |  |
Operating Revenues [Line Items] |  |  |
Total Gas and Electric Operating Revenues | 223.3 | 206.2 | 196.1
Electric | Billed and Unbilled Revenue |  |  |
Operating Revenues [Line Items] |  |  |
Total Gas and Electric Operating Revenues | 228.7 | 201.6 | 188.3
Electric | Rate Adjustment Mechanism Revenue |  |  |
Operating Revenues [Line Items] |  |  |
Total Gas and Electric Operating Revenues | (5.4) | 4.6 | 7.8
Residential | Billed and Unbilled Revenue |  |  |
Operating Revenues [Line Items] |  |  |
Total Gas and Electric Operating Revenues | 205.0 | 179.1 | 163.4
Residential | Gas Segment | Billed and Unbilled Revenue |  |  |
Operating Revenues [Line Items] |  |  |
Total Gas and Electric Operating Revenues | 81.4 | 71.2 | 61.5
Residential | Electric | Billed and Unbilled Revenue |  |  |
Operating Revenues [Line Items] |  |  |
Total Gas and Electric Operating Revenues | 123.6 | 107.9 | 101.9
C&I | Billed and Unbilled Revenue |  |  |
Operating Revenues [Line Items] |  |  |
Total Gas and Electric Operating Revenues | 216.1 | 190.5 | 174.2
C&I | Gas Segment | Billed and Unbilled Revenue |  |  |
Operating Revenues [Line Items] |  |  |
Total Gas and Electric Operating Revenues | 119.7 | 102.8 | 92.7
C&I | Electric | Billed and Unbilled Revenue |  |  |
Operating Revenues [Line Items] |  |  |
Total Gas and Electric Operating Revenues | 96.4 | 87.7 | 81.5
Other | Billed and Unbilled Revenue |  |  |
Operating Revenues [Line Items] |  |  |
Total Gas and Electric Operating Revenues | 24.6 | 19.5 | 16.1
Other | Gas Segment | Billed and Unbilled Revenue |  |  |
Operating Revenues [Line Items] |  |  |
Total Gas and Electric Operating Revenues | 13.3 | 13.5 | 11.2
Other | Electric | Billed and Unbilled Revenue |  |  |
Operating Revenues [Line Items] |  |  |
Total Gas and Electric Operating Revenues | 11.3 | $ 6.0 | $ 4.9
Revenue Reduction - TCJA | Billed and Unbilled Revenue |  |  |
Operating Revenues [Line Items] |  |  |
Total Gas and Electric Operating Revenues | (6.3) |  |
Revenue Reduction - TCJA | Gas Segment | Billed and Unbilled Revenue |  |  |
Operating Revenues [Line Items] |  |  |
Total Gas and Electric Operating Revenues | (3.7) |  |
Revenue Reduction - TCJA | Electric | Billed and Unbilled Revenue |  |  |
Operating Revenues [Line Items] |  |  |
Total Gas and Electric Operating Revenues | $ (2.6) |  |

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