Borrowings - Broker-Dealer Credit Facilities, Narrative (Details)	Nov. 03, 2017 USD ($) borrowing_base	Jan. 31, 2020 USD ($)	Dec. 31, 2019 USD ($) debt_instrument	Mar. 01, 2019 USD ($)	Nov. 02, 2017 USD ($)
Line of Credit Facility [Line Items]
Number of borrowing bases | borrowing_base	2
Broker-dealer credit facilities
Line of Credit Facility [Line Items]
Number of secured credit facilities | debt_instrument			2
Maximum borrowing capacity	$ 500,000,000			$ 600,000,000
Commitment fee (as a percent)	0.50%
Broker-dealer credit facilities | Borrowing Base A Loan | LIBOR
Line of Credit Facility [Line Items]
Interest rate margin (as a percent)	1.25%
Broker-dealer credit facilities | Borrowing Base B Loan | LIBOR
Line of Credit Facility [Line Items]
Interest rate margin (as a percent)	2.50%
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
Minimum | Broker-dealer credit facilities | Borrowing Base A Loan
Line of Credit Facility [Line Items]
Maximum borrowing capacity					$ 300,000,000
Minimum | Broker-dealer credit facilities | Borrowing Base B Loan
Line of Credit Facility [Line Items]
Maximum borrowing capacity					$ 100,000,000
Maximum | Broker-dealer credit facilities | Borrowing Base A Loan
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 600,000,000
Maximum | Broker-dealer credit facilities | Borrowing Base B Loan
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 150,000,000
Subsequent Event | Broker-dealer credit facility on an uncommitted basis
Line of Credit Facility [Line Items]
Maximum borrowing capacity		$ 300,000,000