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Other Commitments And Contingencies (Narrative) (Details) $ in Thousands |  |  | 3 Months Ended | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | ---
Nov. 30, 2018 USD ($) | Jul. 31, 2008 MW | Sep. 30, 2018 | Dec. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2016 USD ($) | May 31, 2018 USD ($) | Feb. 01, 2018 USD ($)
Other Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Payments made in connection with operating leases |  |  |  | $ 49,100 | $ 49,500 | $ 52,000 |  |
Capital lease payments |  |  |  | 213,900 | 171,200 |  |  |
Capital leases related accumulated depreciation |  |  |  | 37,100 | 32,400 |  |  |
Wind power purchase agreement capacity | MW |  | 100 |  |  |  |  |  |
Each wind power purchase agreement capacity | MW |  | 50 |  |  |  |  |  |
Capital Investment Program to Install Over-Pressurization Protection Devices |  |  |  | 150,000 |  |  |  |
Line of Credit Facility, Amount Outstanding |  |  |  | 0 |  |  |  |
Liability Insurance |  |  |  | 800,000 |  |  |  |
Property Insurance |  |  |  | 300,000 |  |  |  |
Insurance Recoveries |  |  |  | 135,000 |  |  |  |
Proceeds From Insurance Settlement |  |  |  | 5,000 |  |  |  |
Estimated Third-Party Claims |  |  |  | 292,000 |  |  |  |
Insurance Settlements Receivable |  |  |  | 130,000 |  |  |  |
Recorded reserves to cover environmental remediation at various sites |  |  |  | 101,200 | 111,400 |  |  |
Payments Pure Air |  |  |  | $ 8,300 | 22,000 |  |  |
Liability Associated with Bailly Generating Station Retirement - Pure Air Contract Termination Portion |  |  |  |  |  | $ 16,000 |  |
MGP Sites |  |  |  |  |  |  |  |
Other Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Number of waste disposal sites identified by program |  |  |  | 63 |  |  |  |
Liability for estimated remediation costs |  |  |  | $ 97,500 | 106,900 |  |  |
Reasonably possible remediation costs variance from reserve |  |  |  | 20,000 |  |  |  |
NIPSCO |  |  |  |  |  |  |  |
Other Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Estimated Cost of Compliance with Coal Combustion Residuals, Maximum |  |  |  | 193,000 |  |  |  |
Bailly Unit 8 Portion of Net Book Value in Net Utility Plant |  |  |  |  |  |  |  | $ 15,000
Estimated Cost of Compliance with Effluent Limitations Guidelines |  |  |  | $ 170,000 |  |  |  |
Regulatory Asset Associated with Bailly Generating Station Retirement |  |  |  |  |  |  | $ 103,000 | $ 142,000
Columbia Of Massachusetts |  |  |  |  |  |  |  |
Other Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Gas Meters Affected |  |  |  | 7,500 |  |  |  |
Businesses Impacted by Incident |  |  |  | 700 |  |  |  |
Consent Order Fine | $ 75 |  |  |  |  |  |  |
Pipeline Replacement Expenses |  |  |  | $ 167,000 |  |  |  |
Expenses Related to Third Party Claims, Other |  |  |  | 266,000 |  |  |  |
Miles of Affected Cast Iron and Bare Steel Pipeline System |  |  | 45 |  |  |  |  |
Loss Recorded During Period |  |  |  | $ 1,023,000 |  |  |  |
Maximum |  |  |  |  |  |  |  |
Other Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Long term purchase commitment time period |  |  |  | 20 years |  |  |  |
Maximum | Columbia Of Massachusetts |  |  |  |  |  |  |  |
Other Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Penalty Per Violation of Operational Directives During Restoration Efforts |  |  |  | $ 1,000 |  |  |  |
Civil Penalty Assessed for Violation of Federal Pipeline Safety Regulations |  |  |  | 209 |  |  |  |
Civil Penalty Assessed for Series of Violations of Federal Pipeline Safety Regulations |  |  |  | 2,100 |  |  |  |
Penalty Per Violation of Emergency Response Plan |  |  |  | 250 |  |  |  |
Penalty for Violations of Emergency Response Plan |  |  |  | 20,000 |  |  |  |
Expenses Related to Third-Party Claims |  |  |  | 790,000 |  |  |  |
Incident-Related Costs |  |  |  | 345,000 |  |  |  |
Estimated Capital Cost of the Pipeline Replacement |  |  |  | $ 230,000 |  |  |  |
Minimum |  |  |  |  |  |  |  |
Other Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Long term purchase commitment time period |  |  |  | 15 years |  |  |  |
Minimum | Columbia Of Massachusetts |  |  |  |  |  |  |  |
Other Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Expenses Related to Third-Party Claims |  |  |  | $ 757,000 |  |  |  |
Incident-Related Costs |  |  |  | 330,000 |  |  |  |
Estimated Capital Cost of the Pipeline Replacement |  |  |  | $ 220,000 |  |  |  |
Pipeline Capacity Transportation And Storage | Maximum |  |  |  |  |  |  |  |
Other Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Long Term Purchase Commitment Expiration Year |  |  |  | 2045 |  |  |  |
Pipeline Capacity Transportation And Storage | Minimum |  |  |  |  |  |  |  |
Other Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Long Term Purchase Commitment Expiration Year |  |  |  | 2019 |  |  |  |
Coal Transportation | Maximum |  |  |  |  |  |  |  |
Other Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Long Term Purchase Commitment Expiration Year |  |  |  | 2021 |  |  |  |
Standby Letters of Credit |  |  |  |  |  |  |  |
Other Commitments And Contingencies [Line Items] |  |  |  |  |  |  |  |
Line of Credit Facility, Amount Outstanding |  |  |  | $ 10,200 | $ 11,100 |  |  |

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