|**Pension Plans And Other Postretirement Benefits (Tables)** | 3 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  
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|Components Of Net Periodic Benefit Costs                     | Pension BenefitsThree Months EndedMarch 31,20202019Service cost$763$680Interest cost2,5402,954Expected return on plan assets(3,938)(3,818)Amortization of prior service cost148155Amortization of actuarial loss1,9921,982Net periodic benefit cost$1,505$1,953OtherPostretirement BenefitsThree Months EndedMarch 31,20202019Service cost$217$205Interest cost677750Expected return on plan assets(675)(621)Amortization of prior service credit(116)(116)Amortization of actuarial loss156166Net periodic benefit cost$259$384
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|Service cost                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Interest cost                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Expected return on plan assets                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Amortization of prior service cost                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Amortization of actuarial loss                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Net periodic benefit cost                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
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|Service cost                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Interest cost                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Expected return on plan assets                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Amortization of prior service credit                         |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Amortization of actuarial loss                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Net periodic benefit cost                                    |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Peoples Gas Acquisition [Member]                             |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Components Of Net Periodic Benefit Costs                     | Three Months EndedMarch 31, 2020Pension BenefitsOther Postretirement BenefitsService cost$40$77Interest cost19254Expected return on plan assets(302)(76)Net periodic benefit (income) cost$(70)$55                                                                                                                                                                                                                                                                                                                              
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|Service cost                                                 |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Interest cost                                                |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Expected return on plan assets                               |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 
|Net periodic benefit (income) cost                           |                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 

X- DefinitionTabular disclosure of the components of net benefit costs for pension plans and/or other employee benefit plans including service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) recognized due to settlements or curtailments.
+ ReferencesReference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920
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