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Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions |  |  |  |  |  |  |  |  |  | 12 Months Ended | 156 Months Ended
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Oct. 31, 2018 | Oct. 25, 2018 | Oct. 09, 2018 | Aug. 28, 2018 | Apr. 18, 2018 | Apr. 13, 2018 | Feb. 13, 2018 | Nov. 08, 2017 | Dec. 27, 2016 | Dec. 31, 2018 | Dec. 31, 2017
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Adjustment |  |  |  |  | $ 5.2 |  |  |  |  |  |  |
Remaining Recovery Period of Regulatory Assets for which No Return on Investment During Recovery Period is Provided |  |  |  |  |  |  |  |  |  |  | 41 years |
Regulatory asset not earning return on investment |  |  |  |  |  |  |  |  |  |  | $ 1,552.6 |
Expenses recovered as components of cost of service and regulatory orders |  |  |  |  |  |  |  |  |  |  | 1,917.1 |
Regulatory assets Requiring Specific Rate Action |  |  |  |  |  |  |  |  |  |  | 320.4 |
Public Utilities, Requested Rate Increase (Decrease), Waiver Amount in excess of Cap |  | $ 2.9 |  |  |  |  |  |  |  |  |  |
Columbia Of Ohio |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount |  |  | $ 74.5 |  |  | $ 70.0 |  |  |  |  |  |
Public Utilities, Requested Biennial Rate Increase (Decrease), Amount increases |  |  |  |  |  | $ 98.0 |  |  |  |  |  |
Depreciation Before Rate Regulation |  |  |  |  |  |  |  |  |  |  |  | $ 806.8
Decrease In Depreciation And Amortization Over That Reflected In Rates |  |  |  |  |  |  |  |  |  |  |  | 92.2
Depreciation Regulatory Asset |  |  |  |  |  |  |  |  |  |  | 39.5 | 49.3
Requested IRP Extension |  |  |  |  |  |  |  |  |  | 5 years |  |
Columbia Of Virginia |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount | [1] |  |  |  | $ 14.2 |  |  |  |  |  |  |
NIPSCO |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Approved Rate Increase (Decrease), Amount |  |  |  |  |  |  |  |  | $ 91.5 |  |  |
Regulatory Net Capital Expenditures Included In Filing |  |  |  |  |  |  |  |  | $ 35.5 |  |  |
Public Utilities, Requested Incremental Capital Investment |  |  |  |  |  |  |  |  |  |  | 228.8 |
Columbia Of Massachusetts |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Public Utilities, Requested Rate Increase (Decrease), Amount |  |  |  |  |  |  | $ 24.1 |  |  |  |  |
Public Utilities, Customer Refund for Tax Rate Change Due to TCJA |  |  |  | $ 95.8 |  |  |  |  |  |  |  |
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) |  |  |  |
IRP | Columbia Of Ohio |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Deferred Depreciation |  |  |  |  |  |  |  |  |  |  | 29.1 | 26.5
Capital Expenditure Program | Columbia Of Ohio |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Deferred Depreciation |  |  |  |  |  |  |  |  |  |  | 76.0 | 49.8
EERM [Domain] | NIPSCO |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Deferred Depreciation |  |  |  |  |  |  |  |  |  |  | 14.4 | 13.9
TDSIC [Domain] | NIPSCO |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Deferred Depreciation |  |  |  |  |  |  |  |  |  |  | 16.5 | 10.3
Deferred Post-in-Service Carrying Charges |  |  |  |  |  |  |  |  |  |  | 9.5 | 8.7
IRP and CEP [Domain] | Columbia Of Ohio |  |  |  |  |  |  |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Deferred Post-in-Service Carrying Charges |  |  |  |  |  |  |  |  |  |  | $ 197.1 | $ 164.6
[1] | Rates implemented subject to refund pending a final order from the VSCC.

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