Contingencies, Commitments and Guarantees Legal Contingencies The Company has been named as a defendant in various legal actions, including complaints and litigation and arbitration claims, arising from its business activities. Such actions include claims related to securities brokerage and investment banking activities, and certain class actions that primarily allege violations of securities laws and seek unspecified damages, which could be substantial. Also, the Company is involved from time to time in investigations and proceedings by governmental agencies and self-regulatory organizations ("SROs") which could result in adverse judgments, settlement, penalties, fines or other relief. The Company has established reserves for potential losses that are probable and reasonably estimable that may result from pending and potential legal actions, investigations and regulatory proceedings. Reasonably possible losses in excess of amounts accrued at December 31, 2018 are not material. In many cases, however, it is inherently difficult to determine whether any loss is probable or even possible or to estimate the amount or range of any potential loss, particularly where proceedings may be in relatively early stages or where plaintiffs are seeking substantial or indeterminate damages. Matters frequently need to be more developed before a loss or range of loss can reasonably be estimated. Given uncertainties regarding the timing, scope, volume and outcome of pending and potential legal actions, investigations and regulatory proceedings and other factors, the amounts of reserves and ranges of reasonably possible losses are difficult to determine and of necessity subject to future revision. Subject to the foregoing, management of the Company believes, based on currently available information, after consultation with outside legal counsel and taking into account its established reserves, that pending legal actions, investigations and regulatory proceedings will be resolved with no material adverse effect on the consolidated statements of financial condition, results of operations or cash flows of the Company. However, if during any period a potential adverse contingency should become probable or resolved for an amount in excess of the established reserves, the results of operations and cash flows in that period and the financial condition as of the end of that period could be materially adversely affected. In addition, there can be no assurance that material losses will not be incurred from claims that have not yet been brought to the Company's attention or are not yet determined to be reasonably possible. Litigation-related reserve activity included within other operating expenses was immaterial for the years ended December 31, 2018 , 2017 and 2016 . Operating Lease Commitments The Company leases office space throughout the United States and in a limited number of foreign countries where the Company's international operations reside. Aggregate minimum lease commitments under operating leases as of December 31, 2018 are as follows: (Dollars in thousands) 2019 $ 13,812 2020 13,686 2021 9,329 2022 7,984 2023 7,143 Thereafter 15,771 Total $ 67,725 Total minimum rentals to be received from 2019 through 2021 under noncancelable subleases were $2.9 million at December 31, 2018 . Rental expense, including operating costs and real estate taxes, was $18.2 million , $17.1 million and $17.3 million for the years ended December 31, 2018 , 2017 and 2016 , respectively. Investment Commitments As of December 31, 2018 , the Company had commitments to invest approximately $78.0 million in limited partnerships or limited liability companies that make direct or indirect equity or debt investments in companies. Other Guarantees The Company is a member of numerous exchanges. Under the membership agreements with these entities, members generally are required to guarantee the performance of other members, and if a member becomes unable to satisfy its obligations to the exchange, other members would be required to meet shortfalls. To mitigate these performance risks, the exchanges often require members to post collateral. In addition, the Company identifies and guarantees certain clearing agents against specified potential losses in connection with providing services to the Company or its affiliates. The Company's maximum potential liability under these arrangements cannot be quantified. However, management believes the likelihood that the Company would be required to make payments under these arrangements is remote. Accordingly, no liability is recorded in the consolidated financial statements for these arrangements. Concentration of Credit Risk The Company provides investment, capital-raising and related services to a diverse group of domestic and foreign customers, including governments, corporations, and institutional and individual investors. The Company's exposure to credit risk associated with the non-performance of customers in fulfilling their contractual obligations pursuant to securities transactions can be directly impacted by volatile securities markets, credit markets and regulatory changes. This exposure is measured on an individual customer basis and on a group basis for customers that share similar attributes. To alleviate the potential for risk concentrations, counterparty credit limits have been implemented for certain products and are continually monitored in light of changing customer and market conditions.