Derivatives And Risk Management (Energy Commodity Derivatives) (Details) frequency in Thousands, Volt in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018 Volt frequency	Dec. 31, 2017 Volt frequency
Sales [Member] | Physical [Member] | Electric Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2018	41	213
2019	123	94
2020	0	0
2021	0	0
2022	0	0
Thereafter	0	0
Sales [Member] | Physical [Member] | Gas Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2018 | Volt	3,101	3,643
2019 | Volt	2,245	1,345
2020 | Volt	1,430	1,430
2021	1,049	1,049
2022	0	0
Thereafter | Volt	0	0
Sales [Member] | Financial [Member] | Electric Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2018	575	1,739
2019	2,403	1,420
2020	836	589
2021	0	0
2022	0	0
Thereafter	0	0
Sales [Member] | Financial [Member] | Gas Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2018 | Volt	20,683	67,375
2019 | Volt	47,488	35,438
2020 | Volt	7,995	915
2021 | Volt	2,275	0
2022	0	0
Thereafter | Volt	0	0
Purchase [Member] | Physical [Member] | Electric Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2018	120	426
2019	204	235
2020	0	0
2021	0	0
2022	0	0
Thereafter	0	0
Purchase [Member] | Physical [Member] | Gas Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2018 | Volt	8,109	10,572
2019 | Volt	5,110	610
2020 | Volt	910	910
2021 | Volt	0	0
2022 | Volt	0	0
Thereafter | Volt	0	0
Purchase [Member] | Financial [Member] | Electric Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2018	542	763
2019	901	737
2020	0	0
2021	0	0
2022	0	0
Thereafter	0	0
Purchase [Member] | Financial [Member] | Gas Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2018 | Volt	34,905	107,580
2019 | Volt	87,118	61,073
2020 | Volt	31,005	16,590
2021 | Volt	4,975	0
2022 | Volt	0	0
Thereafter | Volt	0	0