Operations and Significant Accounting Policies - ALLETE Clean Energy Asset Acquisition (Details) - Caddo [Member] - ALLETE Clean Energy [Member] $ in Millions	3 Months Ended
	Mar. 31, 2020 MW	Mar. 10, 2020 USD ($)
Business Segments [Line Items]
Cash Paid for Assets Acquired | $		$ 8
Generating Capacity Under Development (MW) | MW	300