Nature of Operations And Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions |
12 Months Ended | ||||
|---|---|---|---|---|---|
|
Dec. 31, 2025
USD ($)
|
Dec. 31, 2024
USD ($)
|
Dec. 31, 2023 |
Oct. 28, 2025
Rate
|
Jun. 17, 2023
Rate
|
|
| Basis Of Accounting Presentation [Line Items] | |||||
| Number of customers | 3,800,000 | ||||
| Pre-tax rate for allowance for funds used during construction | 3.60% | 4.80% | 3.90% | ||
| Inventory valued using LIFO | $ 42.5 | $ 43.8 | |||
| Excess of replacement over LIFO value | (7.6) | (12.8) | |||
| Inventory valued using the weighted average cost methodology | $ 209.5 | $ 135.8 | |||
| NIPSCO Holdings II | |||||
| Basis Of Accounting Presentation [Line Items] | |||||
| Subsidiary, Ownership Percentage, Parent | Rate | 19.90% | 19.90% | |||