Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Detail)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
U.S. statutory rate	21.00%	21.00%	21.00%	35.00%
Increases (reductions) resulting from:
State taxes, net of federal benefit	4.20%	3.90%	3.80%
State investment tax credits	0.00%	0.00%	0.30%
AFUDC - equity	(0.10%)	0.00%	0.20%
Amortization of federal investment tax credits	(0.40%)	0.10%	0.20%
Production tax credits	0.00%	0.40%	0.90%
Reversal of excess deferred income taxes	(6.00%)	(1.40%)	0.00%
Federal legislative change	0.00%	0.00%	17.50%
NND Project impairment	0.00%	(2.40%)	(2.30%)
Write-off of regulatory asset	0.00%	(15.80%)	0.00%
Changes in unrecognized tax benefits	0.00%	(5.10%)	0.00%
Other	0.10%	0.20%	(0.20%)
Effective tax rate	18.80%	0.90%	41.40%