Pension Plans And Other Postretirement Benefits (Components Of Net Periodic Benefit Costs) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 763	$ 680	$ 2,289	$ 2,040
Interest cost	2,540	2,954	7,620	8,862
Expected return on plan assets	(3,938)	(3,818)	(11,814)	(11,454)
Amortization of prior service cost	148	155	444	465
Amortization of actuarial loss	1,992	1,982	5,976	5,946
Net periodic benefit (income) cost	1,505	1,953	4,515	5,859
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	217	205	651	615
Interest cost	677	750	2,031	2,250
Expected return on plan assets	(675)	(621)	(2,025)	(1,863)
Amortization of prior service cost	(116)	(116)	(348)	(348)
Amortization of actuarial loss	156	166	468	498
Net periodic benefit (income) cost	259	$ 384	777	$ 1,152
Peoples Gas Acquisition [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	229		494
Interest cost	1,102		2,383
Expected return on plan assets	(1,738)		(3,759)
Net periodic benefit (income) cost	(407)		(882)
Peoples Gas Acquisition [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	445		963
Interest cost	312		676
Expected return on plan assets	(436)		(944)
Net periodic benefit (income) cost	$ 321		$ 695