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Variable Interest Entities - Textual (Details) - Consolidated Limited Partnership-like Entities [Member] - entity
6 Months Ended 12 Months Ended
Jun. 30, 2018
Dec. 31, 2017
Variable Interest Entity [Line Items]    
Number of consolidated limited partnership-like entities 10 10
Minimum [Member]    
Variable Interest Entity [Line Items]    
Ownership interest in consolidated entities (percent) 6.80%  
Maximum [Member]    
Variable Interest Entity [Line Items]    
Ownership interest in consolidated entities (percent) 99.50%