Derivative Financial Instruments - Australia Ravenhall (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 AUD ($)
Derivative [Line Items]
Unrealized gain, net of tax, related to cash flow hedge	$ 2,278,000	$ 1,295,000	$ 3,014,000	$ 816,000
Ravenhall, Australia
Derivative [Line Items]
Ineffective portion of Cash Flow Hedge interest rate swap			0
Ravenhall, Australia | Other Noncurrent Liabilities
Derivative [Line Items]
Fair value of the swap liability	$ 10,400,000		$ 10,400,000
Ravenhall, Australia | Operating phase | Interest Rate Swap
Derivative [Line Items]
Fixed interest rate on derivative	4.20%		4.20%		4.20%
Notional amount coincide with the terms of the non-recourse debt	$ 339,000,000		$ 339,000,000		$ 457
Ravenhall, Australia | Cash Flow Hedge
Derivative [Line Items]
Unrealized gain, net of tax, related to cash flow hedge			$ 3,000,000