Derivatives And Risk Management (Energy Commodity Derivatives) (Details) frequency in Thousands, Volt in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018 Volt frequency	Dec. 31, 2017 Volt frequency
Sales [Member] | Physical [Member] | Electric Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2018	162	213
2019	94	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	7,683	3,643
2019 | Volt	1,345	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	1,537	1,739
2019	1,543	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	58,035	67,375
2019 | Volt	35,438	35,438
2020 | Volt	1,830	915
2021 | Volt	0	0
2022	0	0
Thereafter | Volt	0	0
Purchase [Member] | Physical [Member] | Electric Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2018	246	426
2019	235	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	13,783	10,572
2019 | Volt	610	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	816	763
2019	737	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	86,103	107,580
2019 | Volt	65,478	61,073
2020 | Volt	23,145	16,590
2021 | Volt	0	0
2022 | Volt	0	0
Thereafter | Volt	0	0