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Regulatory Matters (Schedule of Regulatory Programs) (Details) - USD ($) $ in Millions | Apr. 27, 2020 | Apr. 24, 2020 | Apr. 15, 2020 | Feb. 28, 2020 | Feb. 25, 2020 | Jan. 29, 2020 | Nov. 26, 2019 | Oct. 31, 2019 | Oct. 18, 2019 | Oct. 15, 2019 | Aug. 21, 2019 | Aug. 15, 2019 | Jun. 25, 2019 | Feb. 28, 2019 | Oct. 31, 2018
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Columbia Of Ohio | 2020 IRP |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Approved Rate Increase (Decrease), Amount |  |  |  |  | $ 32.9 |  |  |  |  |  |  |  |  |  |  |
Regulatory Net Capital Expenditures Included In Filing |  |  |  |  | 234.4 |  |  |  |  |  |  |  |  |  |  |
Columbia Of Ohio | 2019 CEP |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Approved Rate Increase (Decrease), Amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 15.0 |
Regulatory Net Capital Expenditures Included In Filing |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 121.7 |
Columbia Of Ohio | 2020 CEP |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Approved Rate Increase (Decrease), Amount |  |  |  |  | 18.1 |  |  |  |  |  |  |  |  |  |  |
Regulatory Net Capital Expenditures Included In Filing |  |  |  |  | $ 185.1 |  |  |  |  |  |  |  |  |  |  |
NIPSCO - Gas | TDSIC 10 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Approved Rate Increase (Decrease), Amount | [1] |  |  |  |  |  |  |  |  |  |  |  |  | $ 1.6 |  |
Regulatory Net Capital Expenditures Included In Filing | [1] |  |  |  |  |  |  |  |  |  |  |  |  | $ 12.4 |  |
NIPSCO - Gas | TDSIC 11 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Approved Rate Increase (Decrease), Amount | [2] |  |  |  |  | $ (1.7) |  |  |  |  |  |  |  |  |  |
Regulatory Net Capital Expenditures Included In Filing | [2] |  |  |  |  | $ 38.7 |  |  |  |  |  |  |  |  |  |
NIPSCO - Gas | FMCA 3 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Approved Rate Increase (Decrease), Amount | [3] |  |  |  |  |  |  | $ 0.3 |  |  |  |  |  |  |  |
Regulatory Net Capital Expenditures Included In Filing | [3] |  |  |  |  |  |  | $ 43.0 |  |  |  |  |  |  |  |
Columbia Of Massachusetts | 2020 GSEP |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Approved Rate Increase (Decrease), Amount | [3],[4] |  |  |  |  |  |  |  | $ 0.9 |  |  |  |  |  |  |
Regulatory Net Capital Expenditures Included In Filing | [3],[4] |  |  |  |  |  |  |  | $ 37.5 |  |  |  |  |  |  |
Columbia Of Virginia | 2020 SAVE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount |  |  |  |  |  |  |  |  |  |  |  |  | $ 3.8 |  |  |
Regulatory Net Capital Expenditures Included In Filing |  |  |  |  |  |  |  |  |  |  |  |  | $ 50.0 |  |  |
Columbia Of Kentucky | 2020 SMRP |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount |  |  |  |  |  |  |  |  |  |  | $ 4.2 |  |  |  |  |
Regulatory Net Capital Expenditures Included In Filing |  |  |  |  |  |  |  |  |  |  | $ 40.4 |  |  |  |  |
Columbia Of Maryland | 2020 STRIDE |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Approved Rate Increase (Decrease), Amount |  |  |  |  |  |  | $ 1.3 |  |  |  |  |  |  |  |  |
Regulatory Net Capital Expenditures Included In Filing |  |  |  |  |  |  | $ 15.0 |  |  |  |  |  |  |  |  |
NIPSCO - Electric |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Approved Rate Increase (Decrease), Amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ (53.5)
Public Utilities, Requested Rate Increase (Decrease), Amount |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 21.4
NIPSCO - Electric | TDSIC 6 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount |  |  |  |  |  |  |  |  |  |  |  | $ 28.1 |  |  |  |
Regulatory Net Capital Expenditures Included In Filing |  |  |  |  |  |  |  |  |  |  |  | $ 131.1 |  |  |  |
NIPSCO - Electric | FMCA 12 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Approved Rate Increase (Decrease), Amount | [3] |  |  |  |  |  |  |  |  | $ 1.6 |  |  |  |  |  |
Regulatory Net Capital Expenditures Included In Filing | [3] |  |  |  |  |  |  |  |  | $ 4.7 |  |  |  |  |  |
Subsequent Event | NIPSCO - Electric | FMCA 13 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Approved Rate Increase (Decrease), Amount | [3],[5] |  |  | $ (1.2) |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Net Capital Expenditures Included In Filing | [3],[5] |  |  | $ 0.0 |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event | Columbia Of Pennsylvania [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount |  |  | $ 100.4 |  |  |  |  |  |  |  |  |  |  |  |  |
Subsequent Event | Columbia Of Pennsylvania [Member] | DSIC |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Approved Rate Increase (Decrease), Amount |  | $ 0.9 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Net Capital Expenditures Included In Filing |  | $ 28.2 |  |  |  |  |  |  |  |  |  |  |  |  |  |
[1] | (1)Incremental capital and revenue are net of amounts included in the step 2 rates.
[2] | (2)Incremental revenue is net of amounts included in the step 2 rates and reflects a more typical 6-month filing period.
[3] | (3)Incremental revenue is inclusive of tracker eligible operations and maintenance expense.
[4] | (4)Incremental revenue reflects a 50% decrease in projected 2020 capital investments due to the October 3, 2019 order from the Massachusetts DPU that imposed work restrictions on Columbia of Massachusetts and the Massachusetts DPU's ongoing investigations.
[5] | (5)No eligible capital investments were made during the investment period.

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