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Intangible Assets - Narrative (Details) - USD ($)
3 Months Ended 12 Months Ended
Mar. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
May 24, 2019
Finite-Lived Intangible Assets [Line Items]        
Finite-lived intangible assets acquired   $ 315,400,000    
Weighted average amortization period (years)   7 years 1 month 24 days    
Impairment of intangible asset   $ 0    
Amortization expense $ 15,900,000   $ 20,000,000  
OppenheimerFunds | Management Contracts        
Finite-Lived Intangible Assets [Line Items]        
Finite lived intangible assets acquired       $ 255,000,000.0
OppenheimerFunds | Trade names        
Finite-Lived Intangible Assets [Line Items]        
Finite lived intangible assets acquired       27,000,000.0
OppenheimerFunds | Management Contracts | Management Contracts        
Finite-Lived Intangible Assets [Line Items]        
Indefinite-lived intangible assets acquired       $ 4,907,000,000.0