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Investments - Investment in NGP - Narrative (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2023
Mar. 31, 2022
Schedule of Equity Method Investments [Line Items]    
Amortization period of basis difference 10 years  
Performance allocations, percentage (percent) 47.50%  
NGP Management    
Schedule of Equity Method Investments [Line Items]    
Net investment earnings related to performance allocations $ 1.7 $ 250.4
Net investment (losses) earnings related to principal investment income $ (0.1) $ 18.0
Investment in NGP Management    
Schedule of Equity Method Investments [Line Items]    
Performance allocations, percentage (percent) 47.50%  
Minimum | NGP Management    
Schedule of Equity Method Investments [Line Items]    
Management fee, percentage of commitments (percent) 1.00%  
Management fee, percentage of invested capital (percent) 0.50%  
Maximum | NGP Management    
Schedule of Equity Method Investments [Line Items]    
Management fee, percentage of commitments (percent) 2.00%  
Management fee, percentage of invested capital (percent) 2.00%  
Management Fee Related Revenues | Investment in NGP Management    
Schedule of Equity Method Investments [Line Items]    
Management fee-related revenues, allocation percentage (percent) 55.00%