Accounting for Asset Retirement Obligations (Changes in Estimated Probabilities Assumptions) (Details)	12 Months Ended
	Dec. 31, 2017
Original ARO Liability [Member]
Accounting For Asset Retirement Obligations [Line Items]
Escalation Rate	3.60%
Credit-Risk Adjusted Discount Rate	9.50%
Incremental ARO Liability (2010) [Member]
Accounting For Asset Retirement Obligations [Line Items]
Escalation Rate	3.60%
Credit-Risk Adjusted Discount Rate	6.20%
Incremental ARO Liability (2016) [Member]
Accounting For Asset Retirement Obligations [Line Items]
Escalation Rate	3.25%
Credit-Risk Adjusted Discount Rate	4.34%