Operations and Significant Accounting Policies - Non-Controlling Interest (Details) $ in Millions			6 Months Ended
	Apr. 29, 2020 USD ($)	Apr. 16, 2020 MW	Jun. 30, 2020 USD ($) MW	Jun. 30, 2019 USD ($)
Non-Controlling Interest Details [Line Items]
Proceeds from Noncontrolling Interests | $			$ 67.8	$ 0.0
South Peak [Member]
Non-Controlling Interest Details [Line Items]
Proceeds from Noncontrolling Interests | $	$ 67.8
Generating Capacity Subject to Tax Equity Financing | MW		80	80
Glen Ullin Energy Center [Member]
Non-Controlling Interest Details [Line Items]
Generating Capacity Subject to Tax Equity Financing | MW			106