Derivative Financial Instruments - Interest Rate Swaps (Additional Information) (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Jul. 31, 2015 agreement
Interest rate swap
Credit Derivatives [Line Items]
Gain (loss) on derivative	$ 48	$ 71	$ 95	$ 163
Cash flow hedging | Interest rate swap | PPA Company IIIb
Credit Derivatives [Line Items]
Notional amount	24,300		24,300		$ 24,700
Gain (loss) on derivative	(23)	54
Cash flow hedging | Interest rate swap | PPA Company V
Credit Derivatives [Line Items]
Notional amount	185,800		185,800		$ 186,600
Number of swap agreements entered into | agreement						0
Gain (loss) on derivative	$ 36	$ 55	$ 12	$ 109
Cash flow hedging | Interest rate swap maturing In 2016 | PPA Company V
Credit Derivatives [Line Items]
Number of swap agreements entered into | agreement						0
Cash flow hedging | Interest rate swap maturing September 30, 2031 | PPA Company V
Credit Derivatives [Line Items]
Number of swap agreements entered into | agreement						0
Cash flow hedging | Interest rate swap maturing December 21, 2021 | PPA Company V
Credit Derivatives [Line Items]
Number of swap agreements entered into | agreement						0