Summary of Significant Accounting Policies (Schedule of Weighted Average Depreciation Rates) (Detail)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Generation
Composite weighted average depreciation rates for utility plant	2.50%	2.61%
Transmission
Composite weighted average depreciation rates for utility plant	2.57%	2.74%
Distribution
Composite weighted average depreciation rates for utility plant	2.41%	2.41%
Storage
Composite weighted average depreciation rates for utility plant	2.74%	2.71%
General and other
Composite weighted average depreciation rates for utility plant	3.22%	3.18%