Income Taxes - NPC (Details)	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 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	(2.00%)	(1.00%)	(4.00%)	(3.00%)
Effective income tax rate	12.00%	(1.00%)	(8.00%)	(12.00%)
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	(11.00%)	1.00%	(11.00%)	1.00%
Effective income tax rate	10.00%	22.00%	10.00%	22.00%