Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies - Energy Assets (Details)

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Summary of Significant Accounting Policies - Energy Assets (Details) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accounting Policies [Abstract]      
Interest costs capitalized $ 4,256,000 $ 1,253,000 $ 941,000
Proceeds from Section 1603 grants 0 0 0
Deferred grant income 7,188,000 7,739,000  
Rebates from utility company $ 0 $ 0 $ 0