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Variable Interest Entities - Textuals (Details) - Variable Interest Entity, Primary Beneficiary, Aggregated Disclosure [Member] - entity
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Variable Interest Entity [Line Items]    
Number of consolidated limited partnership-like entities 8 10
Minimum [Member]    
Variable Interest Entity [Line Items]    
Ownership interest in the consolidated entities (percent) 50.00%  
Maximum [Member]    
Variable Interest Entity [Line Items]    
Ownership interest in the consolidated entities (percent) 75.00%