Derivative Financial Instruments (Australia Ravenhall) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 AUD
Derivative [Line Items]
Change in fair value of derivative instrument classified as cash flow hedge	$ 3,985	$ 1,820	$ (1,375)
Payments on non-recourse debt	307,414	$ 10,064	$ 11,908
Interest Rate Swap | Asset under Construction | Ravenhall, Australia
Derivative [Line Items]
Fixed interest rate on cash flow interest rate derivative			3.30%
Notional amount	364,000			AUD 466,300,000
Interest Rate Swap | Operating Phase of Asset | Ravenhall, Australia
Derivative [Line Items]
Fixed interest rate on cash flow interest rate derivative			4.20%
National Australia Bank Limited | Non Recourse Debt | Ravenhall
Derivative [Line Items]
Lump sum due at completion	242,000			AUD 310,000,000
Cash Flow Hedging | Ravenhall, Australia
Derivative [Line Items]
Change in fair value of derivative instrument classified as cash flow hedge	4,000
Other Noncurrent Liabilities | Ravenhall, Australia
Derivative [Line Items]
Value of swap liability	$ 14,000