Description of Business and Summary of Significant Accounting Policies - Schedule of Provision for Depreciation of Electric Plant (Details)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Transmission Plant | Minimum
Public Utility Property Plant And Equipment [Line Items]
Depreciation rate	1.69%	1.69%	1.69%
Transmission Plant | Maximum
Public Utility Property Plant And Equipment [Line Items]
Depreciation rate	3.15%	3.15%	3.15%
Distribution Plant | Minimum
Public Utility Property Plant And Equipment [Line Items]
Depreciation rate	1.74%	1.74%	1.74%
Distribution Plant | Maximum
Public Utility Property Plant And Equipment [Line Items]
Depreciation rate	5.96%	5.96%	5.96%
General Plant | Minimum
Public Utility Property Plant And Equipment [Line Items]
Depreciation rate	0.80%	0.80%	0.80%
General Plant | Maximum
Public Utility Property Plant And Equipment [Line Items]
Depreciation rate	5.12%	5.12%	5.12%