Derivative Financial Instruments - Australia Ravenhall (Details) AUD in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Sep. 30, 2014 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 AUD	Jun. 30, 2016 USD ($)
Derivative [Line Items]
Unrealized losses, net of tax, related to cash flow hedge		$ (2,198,000)	$ 5,097,000	$ (4,642,000)	$ 1,017,000
Payments on non-recourse debt				3,044,000	$ 4,513,000
Ravenhall, Australia
Derivative [Line Items]
Ineffective portion of Cash Flow Hedge interest rate swap				0
Ravenhall, Australia | Interest Rate Cap
Derivative [Line Items]
Payments on non-recourse debt	$ 1,700,000
Ravenhall, Australia | Other Noncurrent Liabilities
Derivative [Line Items]
Fair value of the swap liability							$ 26,200,000
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 483	$ 359,000,000
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	$ 347,000,000
Ravenhall, Australia | Cash Flow Hedge
Derivative [Line Items]
Unrealized losses, net of tax, related to cash flow hedge				$ 4,600,000
Construction Facility | Non-Recourse Debt | National Australia Bank Limited | Ravenhall
Derivative [Line Items]
Lump sum due at completion						AUD 310	$ 231,000,000