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Borrowings - Broker-Dealer Credit Facilities, Narrative (Details)
Nov. 03, 2017
USD ($)
borrowing_base
broker_dealer
Mar. 31, 2019
USD ($)
debt_instrument
broker_dealer
Mar. 01, 2019
USD ($)
Dec. 31, 2018
USD ($)
Nov. 02, 2017
USD ($)
Line of Credit Facility [Line Items]          
Number of borrowing bases | borrowing_base 2        
Number of broker-dealer subsidiaries | broker_dealer 3 5      
Broker-dealer credit facilities          
Line of Credit Facility [Line Items]          
Number of secured credit facilities | debt_instrument   2      
Aggregate borrowing $ 500,000,000 $ 800,000,000 $ 600,000,000 $ 700,000,000  
Commitment fee (as a percent) 0.50%        
Broker-dealer credit facility on an uncommitted basis          
Line of Credit Facility [Line Items]          
Number of secured credit facilities | debt_instrument   1      
Maximum borrowing capacity   $ 200,000,000      
Aggregate borrowing   $ 200,000,000   $ 200,000,000  
Borrowing Base A Loan | Broker-dealer credit facilities          
Line of Credit Facility [Line Items]          
Maximum borrowing capacity $ 600,000,000       $ 400,000,000
Borrowing Base A Loan | Broker-dealer credit facilities | LIBOR          
Line of Credit Facility [Line Items]          
Interest rate margin (as a percent) 1.25%        
Borrowing Base B Loan | Broker-dealer credit facilities          
Line of Credit Facility [Line Items]          
Maximum borrowing capacity $ 150,000,000       $ 40,000,000
Borrowing Base B Loan | Broker-dealer credit facilities | LIBOR          
Line of Credit Facility [Line Items]          
Interest rate margin (as a percent) 2.50%