Pension Plans And Other Postretirement Benefits (Components Of Net Periodic Benefit Costs) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 763	$ 680
Interest cost	2,540	2,954
Expected return on plan assets	(3,938)	(3,818)
Amortization of prior service cost	148	155
Amortization of actuarial loss	1,992	1,982
Net periodic benefit (income) cost	1,505	1,953
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	217	205
Interest cost	677	750
Expected return on plan assets	(675)	(621)
Amortization of prior service cost	(116)	(116)
Amortization of actuarial loss	156	166
Net periodic benefit (income) cost	259	$ 384
Peoples Gas Acquisition [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	40
Interest cost	192
Expected return on plan assets	(302)
Net periodic benefit (income) cost	(70)
Peoples Gas Acquisition [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	77
Interest cost	54
Expected return on plan assets	(76)
Net periodic benefit (income) cost	$ 55