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Income Taxes (Details) - USD ($)
$ in Millions
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Schedule of Effective Income Tax Rate Reconciliation [Line Items]          
Reclassification of long-term income tax receivable     $ 609    
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]          
Federal statutory income tax rate 21.00% 35.00% 21.00% 35.00% 35.00%
Effective Income Tax Rate Reconciliation, Tax Credit, Percent (78.00%) (19.00%) (58.00%) (17.00%)  
State income tax, net of federal income tax benefit (19.00%) 1.00% (25.00%) (2.00%)  
Income tax effect of foreign income (4.00%) (5.00%) (11.00%) (5.00%)  
Effects of ratemaking (8.00%) (0.00%) (8.00%) (0.00%)  
Equity income 1.00% 1.00% 1.00% 1.00%  
Other, net 1.00% (0.00%) (1.00%) (1.00%)  
Effective income tax rate (86.00%) 13.00% (81.00%) 11.00%  
Recognized production tax credits $ 33   $ 62    
Production Tax Credit Carryforwards [Abstract]          
Years eligible for federal renewable energy production tax credit     10 years    
Related Party Tax Expense [Abstract]          
Related party transaction, cash received for income taxes, net     $ 311    
Related party transaction, cash paid for income taxes, net       $ 24  
Long-term income tax receivable (494)   (494)   $ 0
Tax Cuts and Jobs Act of 2017 [Member]          
Schedule of Effective Income Tax Rate Reconciliation [Line Items]          
Increase (decrease) in provisional tax liabilities 20   $ 45    
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation [Abstract]          
Federal statutory income tax rate     21.00%    
Iowa Senate File 2417, Rate Effective Starting 2021 [Member]          
Schedule of Effective Income Tax Rate Reconciliation [Line Items]          
Deferred Tax Liabilities, Net 61   $ 61    
Deferred Income Tax Expense (Benefit)     2    
Parent Company [Member]          
Related Party Tax Expense [Abstract]          
Long-term income tax receivable 494   494    
Long-term income tax receivable [Member]          
Schedule of Effective Income Tax Rate Reconciliation [Line Items]          
Reclassification of long-term income tax receivable     609    
Long-term income tax receivable adjustments     115    
Related Party Tax Expense [Abstract]          
Long-term income tax receivable (494)   (494)    
Long-term income tax receivable [Member] | Iowa Senate File 2417, Rate Effective Starting 2021 [Member]          
Schedule of Effective Income Tax Rate Reconciliation [Line Items]          
Long-term income tax receivable adjustments     $ 115    
Maximum [Member] | Tax Year, Current [Member] | IOWA          
Schedule of Effective Income Tax Rate Reconciliation [Line Items]          
Statutory Income Tax Rate, State and Local, Percent     12.00%    
Maximum [Member] | Iowa Senate File 2417, Rate Effective Starting 2021 [Member] | IOWA          
Schedule of Effective Income Tax Rate Reconciliation [Line Items]          
Statutory Income Tax Rate, State and Local, Percent     9.80%    
Deferred Income Taxes [Member] | Iowa Senate File 2417, Rate Effective Starting 2021 [Member]          
Schedule of Effective Income Tax Rate Reconciliation [Line Items]          
Regulatory Liabilities $ 59   $ 59