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Fair Value Measurements - NPC - Debt (Details) - USD ($)
$ in Millions
Dec. 31, 2021
Dec. 31, 2020
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt $ 49,762 $ 49,866
Level 2    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt, fair value 57,189 60,633
NPC    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt 2,499 2,496
NPC | Level 2    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term debt, fair value $ 3,067 $ 3,245