Fair Value Measurements - NPC - Level 3 (Details) - NPC - Commodity - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Fair Value, Assets (Liabilities), Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 27	$ (38)	$ 15	$ (8)
Changes in fair value recognized in regulatory assets	(6)	(13)	5	(44)
Settlements	4	7	5	8
Ending balance	$ 25	$ (44)	$ 25	$ (44)