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Asset Retirement Obligations (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Sep. 30, 2019
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Asset Retirement Obligation, Revision of Estimate $ 69 $ 82 $ 27 $ 0
Asset Retirement Obligation, Accretion Expense   9 8 7
Asset Retirement Obligation, Liabilities Incurred   0 0 $ 2
Asset Retirement Obligation, Period Increase (Decrease)   $ 14    
Document Period End Date   Dec. 31, 2019    
Decommissioning Liability, Noncurrent   $ 4    
Asset Retirement Obligation, Cash Paid to Settle   4    
Recorded Third-Party Environmental Recoveries, Amount   4 $ 4  
Non Utility [Member] [Member]        
Asset Retirement Obligation, Revision of Estimate $ 13      
Utility Plant [Domain]        
Asset Retirement Obligation, Accretion Expense   4    
Asset Retirement Obligation, Liabilities Incurred   3    
Colstrip [Member]        
Asset Retirement Obligation, Period Increase (Decrease)   $ 11