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Segment Reporting (Narrative) (Details)
9 Months Ended
Sep. 30, 2021
segment
operating_unit
Jun. 30, 2020
Segment Reporting [Abstract]    
Segment reporting, number of segments | segment 3  
Number of distinct operating units | operating_unit 2  
BlackRock, Inc. | Discontinued Operations, Disposed of by Sale    
Schedule of Equity Method Investments [Line Items]    
Equity method investment, ownership percentage   22.40%