Fair Value Measurements - Financial Assets and Liabilities Measured on a Recurring and Nonrecurring Basis (Details) - USD ($) $ in Thousands | Mar. 31, 2024 | Dec. 31, 2023 | Mar. 31, 2023 |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Restricted cash | $ 114,006 | $ 108,180 | $ 105,587 |
Securities owned — trading | 43,428 | 76,088 | |
Investment securities | 57,451 | 91,311 | |
Fair Value, Recurring | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Cash equivalents | | 166 | |
Restricted cash | 110,231 | 103,226 | |
Securities owned — trading | 43,428 | 76,088 | |
Other assets | 941,912 | 858,639 | |
Total assets at fair value | 1,865,578 | 1,758,196 | |
Accounts payable and accrued liabilities | | 118,844 | |
Other Liabilities, Fair Value Disclosure | 284,434 | 296,517 | |
Total liabilities at fair value | 284,434 | 296,517 | |
Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Cash equivalents | | 166 | |
Restricted cash | 110,231 | 103,226 | |
Securities owned — trading | 43,312 | 76,056 | |
Other assets | 937,642 | 854,679 | |
Total assets at fair value | 1,861,192 | 1,754,204 | |
Accounts payable and accrued liabilities | | 0 | |
Other Liabilities, Fair Value Disclosure | 197,172 | 177,673 | |
Total liabilities at fair value | 197,172 | 177,673 | |
Fair Value, Recurring | Fair Value, Inputs, Level 2 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Cash equivalents | | 0 | |
Restricted cash | 0 | 0 | |
Securities owned — trading | 116 | 32 | |
Other assets | 4,270 | 3,960 | |
Total assets at fair value | 4,386 | 3,992 | |
Securities sold, but not yet purchased | | 0 | |
Accounts payable and accrued liabilities | | 0 | |
Other Liabilities, Fair Value Disclosure | 0 | 0 | |
Total liabilities at fair value | 0 | 0 | |
Fair Value, Recurring | Fair Value, Inputs, Level 3 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Cash equivalents | | 0 | |
Restricted cash | 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 | | 118,844 | |
Other Liabilities, Fair Value Disclosure | 87,262 | 118,844 | |
Total liabilities at fair value | 87,262 | 118,844 | |
Equity Securities [Member] | Fair Value, Recurring | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Securities sold, but not yet purchased | 74 | 487 | |
Equity Securities [Member] | Fair Value, Recurring | 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 | 74 | 487 | |
Equity Securities [Member] | Fair Value, Recurring | 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 | 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 | |
Mutual Funds [Member] | Fair Value, Recurring | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Securities sold, but not yet purchased | | 55 | |
Mutual Funds [Member] | Fair Value, Recurring | 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 | | 55 | |
Mutual Funds [Member] | Fair Value, Recurring | 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 | |
Mutual Funds [Member] | Fair Value, Recurring | 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 | |
Securities Sold, Not yet Purchased | Fair Value, Recurring | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Securities sold, but not yet purchased | 79 | 542 | |
Securities Sold, Not yet Purchased | Fair Value, Recurring | 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 | 79 | 542 | |
Securities Sold, Not yet Purchased | Fair Value, Recurring | 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 | |
Securities Sold, Not yet Purchased | Fair Value, Recurring | 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 | |
U.S. Treasury Securities [Member] | Fair Value, Recurring | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Securities sold, but not yet purchased | 5 | | |
U.S. Treasury Securities [Member] | Fair Value, Recurring | 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 | 5 | | |
U.S. Treasury Securities [Member] | Fair Value, Recurring | 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 | | |
U.S. Treasury Securities [Member] | Fair Value, Recurring | 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 | | |
Cash Equivalents Segregated Under Federal Or Other Regulations | Fair Value, Recurring | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Cash equivalents | 769,823 | 720,077 | |
Cash Equivalents Segregated Under Federal Or Other Regulations | Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Cash equivalents | 769,823 | 720,077 | |
Cash Equivalents Segregated Under Federal Or Other Regulations | Fair Value, Recurring | Fair Value, Inputs, Level 2 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Cash equivalents | 0 | 0 | |
Cash Equivalents Segregated Under Federal Or Other Regulations | Fair Value, Recurring | Fair Value, Inputs, Level 3 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Cash equivalents | 0 | 0 | |
Cash Equivalents Not Subject To Segregation | Fair Value, Recurring | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Cash equivalents | 184 | | |
Cash Equivalents Not Subject To Segregation | Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Cash equivalents | 184 | | |
Cash Equivalents Not Subject To Segregation | Fair Value, Recurring | Fair Value, Inputs, Level 2 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Cash equivalents | 0 | | |
Cash Equivalents Not Subject To Segregation | Fair Value, Recurring | Fair Value, Inputs, Level 3 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Cash equivalents | 0 | | |
Money Market Funds [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Securities owned — trading | 118 | 107 | |
Money Market Funds [Member] | Fair Value, Recurring | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Securities owned — trading | 118 | 107 | |
Money Market Funds [Member] | Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Securities owned — trading | 118 | 107 | |
Money Market Funds [Member] | Fair Value, Recurring | 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 | Fair Value, Inputs, Level 3 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Securities owned — trading | 0 | 0 | |
Other Security Investments | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Securities owned — trading | 116 | 32 | |
Other Security Investments | Fair Value, Recurring | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Securities owned — trading | 116 | 32 | |
Other Security Investments | Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Securities owned — trading | 0 | 0 | |
Other Security Investments | Fair Value, Recurring | Fair Value, Inputs, Level 2 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Securities owned — trading | 116 | 32 | |
Other Security Investments | Fair Value, Recurring | 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 | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Securities owned — trading | 14,538 | 50,518 | |
Mutual Funds [Member] | Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Securities owned — trading | 14,538 | 50,518 | |
Mutual Funds [Member] | Fair Value, Recurring | 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 | 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, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Securities owned — trading | 627 | 43 | |
Equity Securities [Member] | Fair Value, Recurring | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Securities owned — trading | 627 | 43 | |
Equity Securities [Member] | Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Securities owned — trading | 627 | 43 | |
Equity Securities [Member] | Fair Value, Recurring | 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 | Fair Value, Inputs, Level 3 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Securities owned — trading | 0 | 0 | |
U.S. Treasury Securities [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Securities owned — trading | 28,029 | 25,388 | |
U.S. Treasury Securities [Member] | Fair Value, Recurring | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Securities owned — trading | 28,029 | 25,388 | |
U.S. Treasury Securities [Member] | Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Securities owned — trading | 28,029 | 25,388 | |
U.S. Treasury Securities [Member] | Fair Value, Recurring | 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 Securities [Member] | Fair Value, Recurring | Fair Value, Inputs, Level 3 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Securities owned — trading | 0 | 0 | |
Deferred Compensation Plan | Fair Value, Recurring | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Other assets | 740,549 | 677,548 | |
Deferred Compensation Plan | Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Other assets | 740,549 | 677,548 | |
Deferred Compensation Plan | Fair Value, Recurring | Fair Value, Inputs, Level 2 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Other assets | 0 | 0 | |
Deferred Compensation Plan | Fair Value, Recurring | Fair Value, Inputs, Level 3 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Other assets | 0 | 0 | |
Other Investments | Fair Value, Recurring | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Other assets | 4,270 | 3,960 | |
Other Investments | Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Other assets | 0 | 0 | |
Other Investments | Fair Value, Recurring | Fair Value, Inputs, Level 2 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Other assets | 4,270 | 3,960 | |
Other Investments | Fair Value, Recurring | Fair Value, Inputs, Level 3 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Other assets | 0 | 0 | |
Fractional Shares | Fair Value, Recurring | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Other assets | 197,093 | 177,131 | |
Other Liabilities, Fair Value Disclosure | 197,093 | 177,131 | |
Fractional Shares | Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Other assets | 197,093 | 177,131 | |
Other Liabilities, Fair Value Disclosure | 197,093 | 177,131 | |
Fractional Shares | Fair Value, Recurring | Fair Value, Inputs, Level 2 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Other assets | 0 | 0 | |
Other Liabilities, Fair Value Disclosure | 0 | | |
Fractional Shares | Fair Value, Recurring | Fair Value, Inputs, Level 3 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Other assets | 0 | $ 0 | |
Other Liabilities, Fair Value Disclosure | 0 | | |
contingent consideration [Domain] | Fair Value, Recurring | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Other Liabilities, Fair Value Disclosure | 87,262 | | |
contingent consideration [Domain] | Fair Value, Recurring | Fair Value, Inputs, Level 1 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Other Liabilities, Fair Value Disclosure | 0 | | |
contingent consideration [Domain] | Fair Value, Recurring | Fair Value, Inputs, Level 2 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Other Liabilities, Fair Value Disclosure | 0 | | |
contingent consideration [Domain] | Fair Value, Recurring | Fair Value, Inputs, Level 3 [Member] | | | |
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] | | | |
Other Liabilities, Fair Value Disclosure | $ 87,262 | | |