Derivative Instruments - Power Purchase Agreements (Details) $ in Thousands	1 Months Ended	12 Months Ended
	Mar. 31, 2019 agreement	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Derivative [Line Items]
Increase in utilities expense		$ 1,200	$ 700	$ 0
Reclassification of other comprehensive income to utilities expense		1,204	749	$ 0
Fair Value		$ (3,926)	(6,693)
Piscataway | Calpine Energy Solutions, LLC
Derivative [Line Items]
Expiration Date		Feb. 28, 2029
Fair Value		$ (2,162)	(2,919)
Chicago | Calpine Energy Solutions, LLC
Derivative [Line Items]
Expiration Date		Feb. 28, 2029
Fair Value		$ (1,764)	$ (3,774)
Power Purchase Agreements
Derivative [Line Items]
Term of agreement	10 years
Power Purchase Agreements | Chicago and Piscataway
Derivative [Line Items]
Number of agreements | agreement	2