Summary of Significant Accounting Policies (Utility Plant) (Details)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Significant Accounting Policies [Line Items]
Average composite depreciation rate	2.27%	2.28%	2.64%
AFUDC rate	5.38%	6.43%	7.18%
Minimum
Significant Accounting Policies [Line Items]
Public Utilities Property, Plant and Equipment, Useful Life	5 years
Maximum
Significant Accounting Policies [Line Items]
Public Utilities Property, Plant and Equipment, Useful Life	48 years