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Summary of Significant Accounting Policies (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Property, Plant and Equipment [Line Items]      
Excise Taxes Collected $ 45 $ 45 $ 43
Regulatory Liabilities 1,421 1,391  
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided 438    
Other Investments $ 26 112  
Document Period End Date Dec. 31, 2019    
Days after the due date 45 days    
Wholesale accounts receivable written off $ 0 0 $ 0
Margin deposit 16 16  
Letters of Credit Outstanding, Amount $ 15 48  
Public Utilities, Allowance for Funds Used During Construction, Rate 7.10%   7.30%
Public Utilities, Allowance for Funds Used During Construction, Capitalized Interest $ 5 6 $ 6
Public Utilities, Allowance for Funds Used During Construction, Capitalized Cost of Equity $ 10 $ 11 $ 12
Depreciation expense rate 3.60% 3.60% 3.60%
Finite-Lived Intangible Assets, Accumulated Amortization $ 366 $ 302  
Amortization of Intangible Assets 64 $ 59 $ 46
Future Amortization Expense, Year One 60    
Future Amortization Expense, Year Two 52    
Future Amortization Expense, Year Three 46    
Future Amortization Expense, Year Four 37    
Future Amortization Expense, Year Five 32    
Power Cost Deadband - Lower Threshold 15    
Power Cost Deadband - Upper Threshold $ 30    
Public Utilities, Approved Return on Equity, Percentage 9.50% 9.50% 9.60%
Quantifying Misstatement in Current Year Financial Statements, Amount $ 3,917 $ 3,838  
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation 4,095 3,803  
Trojan decommissioning [Member]      
Property, Plant and Equipment [Line Items]      
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided 91 25  
Asset Retirement Obligation Costs [Member]      
Property, Plant and Equipment [Line Items]      
Regulatory Liabilities 54 53  
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided 0    
Deferred Income Tax Charge [Member]      
Property, Plant and Equipment [Line Items]      
Regulatory Liabilities 260 $ 267  
Remaining Amounts of Regulatory Assets for which No Return on Investment During Recovery Period is Provided 0    
Previously Reported [Member]      
Property, Plant and Equipment [Line Items]      
Quantifying Misstatement in Current Year Financial Statements, Amount 223    
Public Utilities, Property, Plant and Equipment, Accumulated Depreciation 113    
Accounting Standards Update 2016-02 [Member]      
Property, Plant and Equipment [Line Items]      
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification 44    
Property, Plant and Equipment [Member]      
Property, Plant and Equipment [Line Items]      
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification (178)    
Property, Plant and Equipment [Member] | Building [Member]      
Property, Plant and Equipment [Line Items]      
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification 131    
Property, Plant and Equipment [Member] | Assets Held under Capital Leases [Member]      
Property, Plant and Equipment [Line Items]      
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification 49    
Property, Plant and Equipment [Member] | Accounting Standards Update 2016-02 [Member]      
Property, Plant and Equipment [Line Items]      
New Accounting Pronouncement or Change in Accounting Principle, Effect of Adoption, Quantification 2    
AOCI Attributable to Parent [Member]      
Property, Plant and Equipment [Line Items]      
Reclassification of stranded tax effects due to Tax Reform $ 2