Derivative Financial Instruments - Australia Ravenhall (Details) AUD in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Sep. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2014 USD ($)	Jun. 30, 2015 AUD	Jun. 30, 2015 USD ($)	Sep. 30, 2014 AUD	Sep. 30, 2014 USD ($)
Derivative [Line Items]
Unrealized losses, net of tax, related to cash flow hedge		$ 5,097,000	$ (7,000)	$ 1,017,000	$ 20,000
Payments on non-recourse debt				4,513,000	$ 0
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							$ 357,100,000
Ravenhall, Australia | Interest Rate Cap
Derivative [Line Items]
Payments on non-recourse debt	$ 1,700,000
Loss related to decline in derivative fair value				100,000
Ravenhall, Australia | Other Noncurrent Liabilities
Derivative [Line Items]
Fair value of the swap liability							17,800,000
Ravenhall, Australia | Other Noncurrent Assets | Interest Rate Cap
Derivative [Line Items]
Interest rate swap asset value							$ 400,000
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 162.0	$ 124,000,000
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				$ 1,000,000
Construction Facility | Non-Recourse Debt | National Australia Bank Limited | Ravenhall
Derivative [Line Items]
Lump sum due at completion								AUD 310.0	$ 237,400,000