Commitments We lease office space under operating leases. These leases generally contain fixed contractual rent changes and certain of the leases provide for operating expense reimbursements. We recognize rental expense on operating leases that contain fixed contractual rent changes on a straight line basis over the terms of the respective leases. As of September 30, 2017 , we had 32 leases that expire at various dates through 2025. We incurred rental expense for the fiscal years ended September 30, 2017 , 2016 and 2015 of $4,933 , $4,650 and $4,426 , respectively, including non-cash straight line rent expense of $250 , $328 and $48 , respectively. Rental expense is included in general and administrative expenses in our consolidated statements of comprehensive income. Certain of these leases also provide us with options to extend the respective terms of the leases. The future scheduled minimum lease payments under the terms of these leases as of September 30, 2017 are as follows (per fiscal year ended September 30): 2018 $ 3,929 2019 3,829 2020 3,361 2021 3,214 2022 3,327 Thereafter 9,041 $ 26,701 Some of the foregoing leases are with related parties. As of September 30, 2017 , $23,773 of our future scheduled minimum lease payments are for our principal executive offices, which are leased from an affiliate of ABP Trust pursuant to a ten year lease agreement ending in 2025. For more information about these related party leases, see Note 6, Related Person Transactions . .