Quarterly report pursuant to Section 13 or 15(d)

Investing Receivables (Details)

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Investing Receivables (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Investing receivables $ 55,688 $ 57,493
Investing Receivables    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Fair value of investing receivables 57,000 58,300
Notes Receivable from the City of Huntsville    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Investing receivables $ 52,667 54,472
Notes Receivable from the City of Huntsville | LW Redstone Company, LLC    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Stated interest rate (as a percent) 9.95%  
Other investing loans receivable    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Investing receivables $ 3,021 $ 3,021