Fair Value Measurements - Financial Assets and Liabilities Measured on a Recurring and Nonrecurring Basis (Details) - USD ($) $ in Thousands | Mar. 31, 2021 | Dec. 31, 2020 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | $ 47,964 | $ 29,252 |
Securities sold, but not yet purchased | 1,316 | 206 |
Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 6,529 | 6,205 |
Securities owned — trading | 47,964 | 29,252 |
Other assets | 408,042 | 380,155 |
Total assets at fair value | 462,535 | 415,612 |
Securities sold, but not yet purchased | 1,316 | 206 |
Accounts payable and accrued liabilities | 3,300 | 3,228 |
Total liabilities at fair value | 4,616 | 3,434 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 6,529 | 6,205 |
Securities owned — trading | 47,834 | 29,252 |
Other assets | 398,895 | 371,202 |
Total assets at fair value | 453,258 | 406,659 |
Securities sold, but not yet purchased | 83 | 203 |
Accounts payable and accrued liabilities | 0 | 0 |
Total liabilities at fair value | 83 | 203 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 0 | 0 |
Securities owned — trading | 130 | 0 |
Other assets | 9,147 | 8,953 |
Total assets at fair value | 9,277 | 8,953 |
Securities sold, but not yet purchased | 1,233 | 3 |
Accounts payable and accrued liabilities | 0 | 0 |
Total liabilities at fair value | 1,233 | 3 |
Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Cash equivalents | 0 | 0 |
Securities owned — trading | 0 | 0 |
Other assets | 0 | 0 |
Total assets at fair value | 0 | 0 |
Securities sold, but not yet purchased | 0 | 0 |
Accounts payable and accrued liabilities | 3,300 | 3,228 |
Total liabilities at fair value | 3,300 | 3,228 |
Equity Securities [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities sold, but not yet purchased | 83 | 203 |
Equity Securities [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities sold, but not yet purchased | 83 | 203 |
Equity Securities [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities sold, but not yet purchased | 0 | 0 |
Equity Securities [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities sold, but not yet purchased | 0 | 0 |
Debt Securities [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities sold, but not yet purchased | 1,233 | 3 |
Debt Securities [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities sold, but not yet purchased | 0 | 0 |
Debt Securities [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities sold, but not yet purchased | 1,233 | 3 |
Debt Securities [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities sold, but not yet purchased | 0 | 0 |
Money Market Funds [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 132 | 125 |
Money Market Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 132 | 125 |
Money Market Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 0 | 0 |
Money Market Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 0 | 0 |
Mutual Funds [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 27,692 | 9,137 |
Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 27,692 | 9,137 |
Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 0 | 0 |
Mutual Funds [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 0 | 0 |
Equity Securities [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 512 | 492 |
Equity Securities [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 512 | 492 |
Equity Securities [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 0 | 0 |
Equity Securities [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 0 | 0 |
Debt Securities [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 130 | |
Debt Securities [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 0 | |
Debt Securities [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 130 | |
Debt Securities [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 0 | |
U.S. treasury obligations [Member] | Fair Value, Recurring [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 19,498 | 19,498 |
U.S. treasury obligations [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 1 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 19,498 | 19,498 |
U.S. treasury obligations [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | 0 | 0 |
U.S. treasury obligations [Member] | Fair Value, Recurring [Member] | Fair Value, Inputs, Level 3 [Member] | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | |
Securities owned — trading | $ 0 | $ 0 |