Income Taxes (Details)		3 Months Ended		6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Income Tax Disclosure [Abstract]
Federal statutory rate		21.00%	21.00%	21.00%	21.00%
State tax (net of federal tax effect)		5.10%	5.00%	5.00%	5.00%
Wind PTCs		(21.10%)	(11.90%)	(19.10%)	(10.00%)
Plant regulatory differences	[1]	(7.10%)	(5.50%)	(7.80%)	(5.60%)
Other tax credits, net NOL & tax credit allowances		(1.90%)	(0.60%)	(1.40%)	(1.80%)
Other (net)		(0.70%)	1.20%	(1.10%)	(0.50%)
Effective income tax rate		(4.70%)	9.20%	(3.40%)	8.10%

[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.