Summary of Significant Accounting Policies (Utility Plant) (Details)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Significant Accounting Policies [Line Items]
Average composite depreciation rate	2.28%	2.64%	2.60%
AFUDC rate	6.43%	7.18%	8.15%
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