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Regulatory Matters (Rate Case Action) (Details) - USD ($) $ in Millions | Oct. 31, 2018 | Aug. 28, 2018 | Apr. 13, 2018 | Mar. 16, 2018 | Feb. 13, 2018 | Sep. 27, 2017
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NIPSCO - Gas |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount | [1] |  |  |  |  |  | $ 138.1
Public Utilities, Approved Rate Increase (Decrease), Amount | [1] |  |  |  |  |  | $ 107.3
Columbia Of Pennsylvania |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount |  |  |  |  | $ 46.9 |  |
Public Utilities, Approved Rate Increase (Decrease), Amount |  |  |  |  | $ 26.0 |  |
Columbia Of Virginia |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount | [2] |  | $ 14.2 |  |  |  |
Columbia Of Maryland |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount |  |  |  | $ 4.6 |  |  |
Public Utilities, Approved Rate Increase (Decrease), Amount |  |  |  | 2.2 |  | $ (1.3) |
Columbia Of Massachusetts |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount |  |  |  | $ 24.1 |  |  |
NIPSCO - Electric |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount |  | $ 21.4 |  |  |  |  |
[1] | Rates will be implemented in three steps, with implementation of step 1 rates effective October 1, 2018. Step 2 rates will be effective on or about March 1, 2019, and step 3 rates will be effective on January 1, 2020. The IURC’s order also dismissed NIPSCO from phase 2 of the IURC’s TCJA investigation.
[2] | Rates implemented subject to refund pending a final order from the VSCC.

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