Derivative Financial Instruments - Interest Rate Swaps (Additional Information) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jul. 31, 2015 agreement
Interest rate swap
Credit Derivatives [Line Items]
Gain (loss) on derivative	$ 206	$ 191
Cash flow hedging | Interest rate swap | PPA Company IIIb
Credit Derivatives [Line Items]
Notional amount	24,700	25,600
Gain (loss) on derivative	(68)	(64)	$ (57)
Cash flow hedging | Interest rate swap | PPA Company V
Credit Derivatives [Line Items]
Notional amount	186,600	188,500
Gain (loss) on derivative	$ 138	$ 126	$ 72
Number of swap agreements entered into | agreement				9
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 September 30, 2031 | 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