Derivatives And Risk Management (Energy Commodity Derivatives) (Details) frequency in Thousands, Volt in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019 Volt frequency	Dec. 31, 2018 Volt frequency
Sales [Member] | Physical [Member] | Electric Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2019	109	197
2020	123	123
2021	0	0
2022	0	0
2023	0	0
Thereafter	0	0
Sales [Member] | Physical [Member] | Gas Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2019 | Volt	669	2,909
2020 | Volt	1,430	1,430
2021 | Volt	1,049	1,049
2022	0	0
2023	0	0
Thereafter | Volt	0	0
Sales [Member] | Financial [Member] | Electric Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2019	2,280	2,790
2020	959	959
2021	246	0
2022	0	0
2023	0	0
Thereafter	0	0
Sales [Member] | Financial [Member] | Gas Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2019 | Volt	35,913	54,418
2020 | Volt	16,378	14,625
2021 | Volt	4,100	4,100
2022 | Volt	0	0
2023	0	0
Thereafter | Volt	0	0
Purchase [Member] | Physical [Member] | Electric Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2019	30	206
2020	0	0
2021	0	0
2022	0	0
2023	0	0
Thereafter	0	0
Purchase [Member] | Physical [Member] | Gas Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2019 | Volt	4,640	10,732
2020 | Volt	1,138	1,138
2021 | Volt	0	0
2022 | Volt	0	0
2023 | Volt	0	0
Thereafter | Volt	0	0
Purchase [Member] | Financial [Member] | Electric Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2019	596	941
2020	0	0
2021	0	0
2022	0	0
2023	0	0
Thereafter	0	0
Purchase [Member] | Financial [Member] | Gas Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2019 | Volt	81,438	101,293
2020 | Volt	53,388	47,225
2021 | Volt	15,290	9,670
2022 | Volt	0	0
2023 | Volt	0	0
Thereafter | Volt	0	0