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Long-term Debt - Series E,F,G,H Notes (Details)
$ in Millions
Jun. 19, 2018
USD ($)
Energy Center Minneapolis Series E Notes  
Debt Instrument [Line Items]  
Proceeds from energy center notes $ 70
Interest rate, percentage 4.80%
Energy Center Minneapolis Series F Notes  
Debt Instrument [Line Items]  
Proceeds from energy center notes $ 10
Interest rate, percentage 4.60%
Energy Center Minneapolis Series G Notes  
Debt Instrument [Line Items]  
Proceeds from energy center notes $ 83
Interest rate, percentage 5.90%
Energy Center Minneapolis Series H Notes  
Debt Instrument [Line Items]  
Proceeds from energy center notes $ 40
Interest rate, percentage 4.83%
Energy Center Minneapolis Series E, F, G, H Notes  
Debt Instrument [Line Items]  
Proceeds from energy center notes $ 203
Net borrowings $ 120
Energy Center Minneapolis Series C Notes  
Debt Instrument [Line Items]  
Interest rate, percentage 5.95%
Aggregate principal amount repurchased $ (83)