Fair Value Measurements - Financial Instruments Not Measured at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Financial Assets:
Cash and cash equivalents	$ 1,389,876	$ 1,231,593	$ 1,246,537
Deposits with banks and short-term investments	1,134,463	1,180,686	1,006,969
Cash deposited with clearing organizations and other segregated cash	44,488	43,280	$ 38,379
Interest-bearing financing receivables	90,521	77,052
Financial Liabilities:
Deposits and other customer payables	1,201,150	1,246,200
Senior debt	1,682,741	1,679,562
Carrying Value [Member]
Financial Assets:
Cash and cash equivalents	1,389,876	1,231,593
Deposits with banks and short-term investments	1,134,463	1,180,686
Cash deposited with clearing organizations and other segregated cash	44,488	43,280
Interest-bearing financing receivables	90,521	77,052
Interest-bearing deposits (included within investments)		517
Financial Liabilities:
Deposits and other customer payables	1,201,150	1,246,200
Senior debt	1,682,741	1,679,562
Fair Value [Member]
Financial Assets:
Deposits with banks and short-term investments	1,134,463	1,180,686
Cash deposited with clearing organizations and other segregated cash	44,488	43,280
Interest-bearing financing receivables	92,584	78,940
Cash and cash equivalents	1,389,876	1,231,593
Interest-bearing deposits (included within investments)		517
Financial Liabilities:
Deposits and other customer payables	1,201,150	1,246,200
Senior debt	1,954,145	1,838,716
Fair Value [Member] \| Level 1 [Member]
Financial Assets:
Deposits with banks and short-term investments	1,134,463	1,180,686
Cash deposited with clearing organizations and other segregated cash	44,488	43,280
Cash and cash equivalents	1,389,876	1,231,593
Interest-bearing deposits (included within investments)		517
Financial Liabilities:
Deposits and other customer payables	1,201,150	1,246,200
Fair Value [Member] \| Level 2 [Member]
Financial Liabilities:
Senior debt	1,954,145	1,838,716
Fair Value [Member] \| Level 3 [Member]
Financial Assets:
Interest-bearing financing receivables	$ 92,584	$ 78,940

Certificates of deposit at fair value.

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Deposits and other customer payables principally related to customer-related interest-bearing time and demand deposits, short-term inter-bank borrowing with registered banks, amounts due on collateralized borrowing activities and other related party payables.

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Represents deposits with banks and investments with a maturity of less than 90 days.

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Amount of interest bearing receivables included in total fee receivables.

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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Amount of cash subject to withdrawal restrictions, restricted deposits held as compensating balances, and cash and securities segregated in compliance with regulations (such as cash deposited in a special reserve account for the exclusive benefit of customers).

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A savings certificate entitling the Entity (that is, bearer) to receive interest at an established maturity date, based upon a fixed interest rate. A certificate of deposit may be issued in any denomination. Certificates of deposit are generally issued by commercial banks and, therefore, insured by the FDIC (up to the prescribed limit). Certificates of deposit generally restrict holders from withdrawing funds on demand without the incurrence of penalties. Generally, only certificates of deposit with original maturities of three months or less qualify as cash equivalents. Original maturity means original maturity to the entity holding the investment. As a related example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three-years ago does not become a cash equivalent when its remaining maturity is three months.

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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