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Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions | Apr. 26, 2017 | Apr. 14, 2017 | Mar. 17, 2017 | Oct. 31, 2016 | Jun. 30, 2016 | Apr. 29, 2016 | Dec. 31, 2016 | Dec. 28, 2016
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NIPSCO |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount |  |  |  |  | $ 45.5 |  |  |
NIPSCO | TDSIC 5 Gas |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Regulatory Net Capital Expenditures Included In Filing |  |  |  |  | $ 211.6 |  |  |
Seven Year Plan of Eligible Investments Under TDSIC |  |  |  |  |  |  |  | $ 824.0
NIPSCO | TDSIC 6 Gas |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Regulatory Net Capital Expenditures Included In Filing |  |  |  |  |  |  | $ 271.3 |
NIPSCO | ECR 29 |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Regulatory Net Capital Expenditures Included In Filing |  |  |  |  |  |  | $ 261.1 |
Columbia Of Massachusetts | 2017 GSEP |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Cumulative Amount |  |  |  | $ 17.2 |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount |  |  |  | $ 8.4 |  |  |  |
Columbia Of Virginia |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount |  |  |  |  |  | $ 37.0 |  |
Public Utilities, Approved Rate Increase (Decrease), Amount |  |  | $ 28.5 |  |  |  |  |
Subsequent Event | Columbia Of Ohio |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Public Utilities, Approved Rate Increase (Decrease), Amount | $ 31.5 |  |  |  |  |  |  |
Subsequent Event | Columbia Of Maryland |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amended, Amount |  | $ 6.0 |  |  |  |  |  |

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