Derivative Instruments And Hedging Activities Volumes related to price risk purchase derivatives (Details) - Natural Gas - long [Member] - Price Risk Derivative [Member]	6 Months Ended
	Jun. 30, 2017 MMBTU
Price Risk Derivatives for Future Purchases Notional Contract Volumes [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure	12,000,000
Maturing in Next Twelve Months [Member]
Price Risk Derivatives for Future Purchases Notional Contract Volumes [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure	4,800,000
Maturing in Year Two [Member]
Price Risk Derivatives for Future Purchases Notional Contract Volumes [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure	1,800,000
Maturing in Year Five [Member]
Price Risk Derivatives for Future Purchases Notional Contract Volumes [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure	1,800,000
Maturing in Year Four [Member]
Price Risk Derivatives for Future Purchases Notional Contract Volumes [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure	1,800,000
Maturing in Year Three [Member]
Price Risk Derivatives for Future Purchases Notional Contract Volumes [Line Items]
Derivative, Nonmonetary Notional Amount, Energy Measure	1,800,000