Income Taxes (Schedule of Effective Income Tax Rate Reconciliation) (Detail)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Income Tax Disclosure [Abstract]
U.S. statutory rate	21.00%	21.00%	21.00%
Increases (reductions) resulting from:
State taxes, net of federal benefit	5.60%	4.20%	3.90%
AFUDC - equity	(0.40%)	(0.10%)	0.00%
Amortization of federal investment tax credits	(0.60%)	(0.40%)	0.10%
Production tax credits	0.00%	0.00%	0.40%
Reversal of excess deferred income taxes	(8.10%)	(6.00%)	(1.40%)
NND Project impairment	0.00%	0.00%	(2.40%)
Write-off of regulatory asset	0.00%	0.00%	(15.80%)
Changes in unrecognized tax benefits	(15.80%)	0.00%	(5.10%)
Prior period adjustments	1.60%
Other	0.50%	0.10%	0.20%
Effective tax rate	3.80%	18.80%	0.90%