Derivative Financial Instruments - Interest Rate Swaps (Additional Information) (Details) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Jul. 31, 2015 agreement
Interest rate swap
Credit Derivatives [Line Items]
Gain (loss) on derivative			$ 163		$ 190
Cash flow hedging | Interest rate swap | PPA Company IIIb
Credit Derivatives [Line Items]
Notional amount	$ 25,200		25,200		25,600
Gain (loss) on derivative	17	$ (14)	54	$ (30)
Cash flow hedging | Interest rate swap | PPA Company V
Credit Derivatives [Line Items]
Notional amount	187,900		187,900		$ 188,100
Gain (loss) on derivative	$ 55	$ 20	$ 109	$ 53
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