Short-term Debt and Credit Facilities - NPC (Details) - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Line of Credit Facility [Line Items]
Credit facilities	$ 11,281	$ 11,030
Short-term debt	(2,009)	(2,286)
Net credit facilities	8,683	8,154
NPC
Line of Credit Facility [Line Items]
Credit facilities	400	400
Short-term debt	(180)	0
Net credit facilities	220	400
Letters of credit outstanding	15
NPC | Line of credit
Line of Credit Facility [Line Items]
Short-term debt	$ (180)	$ 0
Weighted average interest rate	0.86%
NPC | Secured credit facility, $400 million, expiring June 2024 | Line of credit
Line of Credit Facility [Line Items]
Credit facilities	$ 400
Debt to capitalization ratio	0.65