Fair Value Measurements - NPC - Level 3 (Details) - NPC - Commodity - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Fair Value, Assets (Liabilities), Net, Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 25	$ (44)	$ 15	$ (8)
Changes in fair value recognized in regulatory assets	6	13	11	(31)
Settlements	(45)	31	(40)	39
Ending balance	$ (14)	$ 0	$ (14)	$ 0