Pension Plan and Retirement Benefits - Defined Contribution Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
401(k)
Defined Contribution Plans
Expenses associated with plan	$ 1,811	$ 1,590	$ 1,652
401(k) | Employees with exceptions
Defined Contribution Plans
Percentage employer matches of the employee's percentage contribution matched.	50.00%
401(k) | Employees with exceptions | Maximum
Defined Contribution Plans
Percentage of employees' gross pay for which the employer contributes a matching contribution	3.00%
401(k) | Certain employees not eligible for U.S. pension plan
Defined Contribution Plans
Percentage employer matches of the employee's percentage contribution matched.	60.00%
401(k) | Certain employees not eligible for U.S. pension plan | Maximum
Defined Contribution Plans
Percentage of employees' gross pay for which the employer contributes a matching contribution	6.00%
Profit sharing plans
Defined Contribution Plans
Expenses associated with plan	$ 0	$ 0	$ 0