Derivative Financial Instruments - Australia Ravenhall (Details) AUD in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Sep. 30, 2015 AUD	Sep. 30, 2015 USD ($)	Sep. 30, 2014 AUD	Sep. 30, 2014 USD ($)
Derivative [Line Items]
Unrealized losses, net of tax, related to cash flow hedge		$ (3,470,000)	$ (4,877,000)	$ (2,453,000)	$ (4,858,000)
Payments on non-recourse debt				6,366,000	$ 4,511,000
Ravenhall, Australia
Derivative [Line Items]
Ineffective portion of Cash Flow Hedge interest rate swap				0
Ravenhall, Australia | Interest Rate Swap
Derivative [Line Items]
Notional amount coincide with the terms of the non-recourse debt							$ 325,400,000
Ravenhall, Australia | Interest Rate Cap
Derivative [Line Items]
Payments on non-recourse debt	$ 1,700,000
Loss related to decline in derivative fair value				300,000
Ravenhall, Australia | Other Noncurrent Liabilities
Derivative [Line Items]
Fair value of the swap liability							22,000,000
Ravenhall, Australia | Other Noncurrent Assets | Interest Rate Cap
Derivative [Line Items]
Interest rate swap asset value							$ 200,000
Ravenhall, Australia | Design and construction phase | Interest Rate Swap
Derivative [Line Items]
Fixed interest rate on derivative						3.34%	3.34%
Notional amount coincide with the terms of the non-recourse debt						AUD 188.9	$ 131,800,000
Ravenhall, Australia | Operating phase | Interest Rate Swap
Derivative [Line Items]
Fixed interest rate on derivative						4.21%	4.21%
Notional amount coincide with the terms of the non-recourse debt | AUD						AUD 466.3
Ravenhall, Australia | Cash Flow Hedge
Derivative [Line Items]
Unrealized losses, net of tax, related to cash flow hedge				$ 2,500,000
Construction Facility | Non-Recourse Debt | National Australia Bank Limited | Ravenhall
Derivative [Line Items]
Lump sum due at completion								AUD 310.0	$ 216,300,000