Fair Value Measurements - NPC - Level 3 (Details) - NPC - Commodity Derivatives - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 15	$ (8)	$ 3
Changes in fair value recognized in net regulatory assets	(90)	(17)	(21)
Settlements	(38)	40	10
Ending balance	$ (113)	$ 15	$ (8)