Derivative Financial Instruments - Interest Rate Swaps (Additional Information) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jul. 31, 2015 agreement
Credit Derivatives [Line Items]
Gain (loss) on derivative	$ 600	$ (2,200)	$ 200
Interest rate swap
Credit Derivatives [Line Items]
Gain (loss) on derivative	145	225
Natural gas forward contract | Not designated as hedging instrument
Credit Derivatives [Line Items]
Gain (loss) on derivative	(100)	(800)
Gain on the settlement of contracts	4,500	3,600
Cash flow hedging | Interest rate swap | PPA Company V
Credit Derivatives [Line Items]
Number of swap agreements entered into | agreement				9
Derivative, notional amount	$ 181,400	$ 184,200	$ 186,600
Cash flow hedging | Interest rate swap maturing In 2016 | PPA Company V
Credit Derivatives [Line Items]
Number of swap agreements entered into | agreement				3
Cash flow hedging | Interest rate swap maturing December 21, 2021 | PPA Company V
Credit Derivatives [Line Items]
Number of swap agreements entered into | agreement				3
Cash flow hedging | Interest rate swap maturing September 30, 2031 | PPA Company V
Credit Derivatives [Line Items]
Number of swap agreements entered into | agreement				3