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Income Taxes (Details) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Income Tax Disclosure [Abstract]        
Federal statutory rate 21.00% 21.00% 21.00% 21.00%
State tax (net of federal tax effect) 5.00% 5.00% 5.10% 5.00%
Wind PTCs (8.00%) (6.10%) (13.20%) (8.10%)
Plant regulatory differences [1] (7.20%) (5.60%) (7.40%) (5.50%)
NOL carryback (1.00%) (1.70%) (1.20%) (1.80%)
Other (net) (1.20%) (0.60%) (1.30%) (0.50%)
Effective income tax rate 6.70% 12.00% 2.00% 10.10%
Tax Adjustments, Settlements, and Unusual Provisions   $ 13    
Income Tax Examination [Line Items]        
Federal income tax adjustment $ 0   $ 0  
Effective Income Tax Rate Reconciliation, Net Operating Loss Carryback, Percent (1.90%) 0.00% (1.00%) 0.00%
[1] Regulatory differences for income tax primarily relate to the credit of excess deferred taxes to customers through the average rate assumption method. Income tax benefits associated with the credit of excess deferred credits are offset by corresponding revenue reductions.