Employee Benefit Plans	12 Months Ended
Sep. 30, 2020
Compensation And Retirement Disclosure [Abstract]
Employee Benefit Plans	18. Employee Benefit Plans The Company maintains defined contribution plans covering all full-time employees of the Company who have 90 days of service and are at least 21 years old. An eligible employee may elect to make a before-tax contribution of between 1% and 100% of his or her compensation through payroll deductions, not to exceed the annual limit set by law. The Company currently matches the first 50% of participant contributions limited to 6% of a participant’s gross compensation (maximum Company match is 3%). The combined total expense for this plan and a similar plan for Canadian employees was $12.1 million, $11.7 million, and $11.8 million for the years ended September 30, 2020, 2019, and 2018, respectively. The Company sponsors an external pension fund for certain of its foreign employees who belong to a local union. Pension contributions are made to government-sponsored social security pension plans in accordance with local legal requirements. Annual contributions were $1.7 million, $1.0 million, and $0.2 million for the years ended September 30, 2020, 2019, and 2018, respectively. The Company also participates in multi-employer defined benefit plans for which it is not the sponsor. The aggregated expense for these plans was $2.5 million, $2.6 million, and $1.8 million for the years ended September 30, 2020, 2019, and 2018, respectively. Withdrawal from participation in one of these plans requires the Company to make a lump-sum contribution to the plan, and the Company’s withdrawal liability depends on the extent of the plan’s funding of vested benefits, among other factors. During the year ended September 30, 2020, the Company withdrew from the Central States Pension Fund and Local 408 Pension Fund, both of which were reported to have underfunded liabilities. The Company reached a settlement agreement with each fund, and the lump-sum contributions made to exit the funds did not have a material impact on its results of operations.

Employee Benefit Plans

12 Months Ended

Sep. 30, 2020

Compensation And Retirement Disclosure [Abstract]

18. Employee Benefit Plans

The Company maintains defined contribution plans covering all full-time employees of the Company who have 90 days of service and are at least 21 years old. An eligible employee may elect to make a before-tax contribution of between 1% and 100% of his or her compensation through payroll deductions, not to exceed the annual limit set by law. The Company currently matches the first 50% of participant contributions limited to 6% of a participant’s gross compensation (maximum Company match is 3%). The combined total expense for this plan and a similar plan for Canadian employees was $12.1 million, $11.7 million, and $11.8 million for the years ended September 30, 2020, 2019, and 2018, respectively.

The Company sponsors an external pension fund for certain of its foreign employees who belong to a local union. Pension contributions are made to government-sponsored social security pension plans in accordance with local legal requirements. Annual contributions were $1.7 million, $1.0 million, and $0.2 million for the years ended September 30, 2020, 2019, and 2018, respectively.

The Company also participates in multi-employer defined benefit plans for which it is not the sponsor. The aggregated expense for these plans was $2.5 million, $2.6 million, and $1.8 million for the years ended September 30, 2020, 2019, and 2018, respectively. Withdrawal from participation in one of these plans requires the Company to make a lump-sum contribution to the plan, and the Company’s withdrawal liability depends on the extent of the plan’s funding of vested benefits, among other factors. During the year ended September 30, 2020, the Company withdrew from the Central States Pension Fund and Local 408 Pension Fund, both of which were reported to have underfunded liabilities. The Company reached a settlement agreement with each fund, and the lump-sum contributions made to exit the funds did not have a material impact on its results of operations.