Derivative Financial Instruments - Australia Ravenhall (Details) AUD in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 AUD	Jun. 30, 2017 USD ($)
Derivative [Line Items]
Unrealized losses, net of tax, related to cash flow hedge	$ (1,295,000)	$ 2,198,000	$ (816,000)	$ 4,642,000
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						$ 17,700,000
Ravenhall, Australia | Design and construction phase | Interest Rate Swap
Derivative [Line Items]
Fixed interest rate on derivative					3.30%	3.30%
Notional amount coincide with the terms of the non-recourse debt					AUD 671	$ 516,000,000
Ravenhall, Australia | Operating phase | Interest Rate Swap
Derivative [Line Items]
Fixed interest rate on derivative					4.20%	4.20%
Notional amount coincide with the terms of the non-recourse debt					AUD 466	$ 358,000,000
Ravenhall, Australia | Cash Flow Hedge
Derivative [Line Items]
Unrealized losses, net of tax, related to cash flow hedge			$ 800,000
Construction Facility | Non-Recourse Debt | National Australia Bank Limited | Ravenhall
Derivative [Line Items]
Lump sum due at completion					AUD 310	$ 238,000,000