Common Equity - Dividend Restrictions (Details) - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Dividend Payment Restrictions [Line Items]
Restricted retained earnings	$ 2.2
$100 par value, Serial Preferred Stock, 3.60% Series
Dividend Payment Restrictions [Line Items]
Preferred Stock, dividend rate (as a percent)	3.60%	3.60%
Minimum | $100 par value, Serial Preferred Stock, 3.60% Series | Common stock equity to total capitalization is between 25% and 20%
Dividend Payment Restrictions [Line Items]
Percentage of common equity to total capitalization required to be maintained	20.00%
Maximum | $100 par value, Serial Preferred Stock, 3.60% Series | Common stock equity to total capitalization is between 25% and 20%
Dividend Payment Restrictions [Line Items]
Percentage of net income for which dividends can be declared	75.00%
Percentage of common equity to total capitalization required to be maintained	25.00%
Maximum | $100 par value, Serial Preferred Stock, 3.60% Series | Common stock equity to total capitalization is less than 20%
Dividend Payment Restrictions [Line Items]
Percentage of net income for which dividends can be declared	50.00%
Percentage of common equity to total capitalization required to be maintained	20.00%
Public Service Commission of Wisconsin | Minimum
Dividend Payment Restrictions [Line Items]
Common equity ratio required to be maintained (as a percent)	52.50%