Retirement Benefit Plans - Net Pension Expense for Defined Benefit Plans (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Components of net periodic benefit cost
Service cost	$ 262	$ 324
Interest cost	963	883
Pension Plan | United States
Components of net periodic benefit cost
Service cost	262	324
Interest cost	963	883
Expected return on plan assets	(1,608)	(1,615)
Amortization of net loss	641	861
Net pension expense for defined benefit plan	$ 258	$ 453