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Summary of Significant Accounting Policies and Practices - Noncontrolling Interest (Details)
Jan. 01, 2016
Feb. 07, 2018
Dec. 31, 2017
Dec. 31, 2015
Jan. 01, 2012
Noncontrolling Interest [Line Items]          
Ownership percentage sold 1.00%        
Whitetail Rock [Member]          
Noncontrolling Interest [Line Items]          
Noncontrolling interest, ownership percentage         10.00%
ALLO Communications [Member]          
Noncontrolling Interest [Line Items]          
Noncontrolling interest, ownership percentage     7.50%    
Percentage of voting interests acquired       92.50%  
Noncontrolling Interest, minimum earn up percentage by noncontrolling owners     11.50%    
Maximum earn up percentage by noncontrolling owners     19.00%    
401 Building, LLC [Member]          
Noncontrolling Interest [Line Items]          
Ownership percentage by parent     50.00%    
TDP Phase Three, LLC [Member]          
Noncontrolling Interest [Line Items]          
Ownership percentage by parent     25.00%    
330-333 Building, LLC [Member]          
Noncontrolling Interest [Line Items]          
Ownership percentage by parent     50.00%    
GreatNet, LLC [Member]          
Noncontrolling Interest [Line Items]          
Ownership percentage by parent     50.00%    
Subsequent Event [Member] | Great Lakes Educational Loan Services [Member]          
Noncontrolling Interest [Line Items]          
Percentage of voting interests acquired   100.00%