Income Taxes - NPC (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Schedule of Effective Income Tax Rate Reconciliation [Line Items]
Federal statutory income tax rate	21.00%	21.00%	21.00%	21.00%
Effects of ratemaking	(6.00%)	(2.00%)	(5.00%)	(2.00%)
Effective income tax rate	(21.00%)	3.00%	(13.00%)	(2.00%)
NPC
Schedule of Effective Income Tax Rate Reconciliation [Line Items]
Federal statutory income tax rate	21.00%	21.00%	21.00%	21.00%
Effects of ratemaking	(10.00%)	0.00%	(10.00%)	0.00%
Effective income tax rate	11.00%	21.00%	11.00%	21.00%