Pension Plan and Retirement Benefits - Defined Benefit Plans - Components (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Pension Plans
Components of net periodic pension cost and postretirement health care benefit cost
Service cost	$ 5,536	$ 6,282	$ 4,080
Interest cost	10,801	10,577	12,050
Expected return	(15,157)	(14,419)	(14,380)
Amortization of prior service cost	374	808	808
Recognized actuarial loss	4,910	11,267	8,838
Net periodic benefit cost	6,464	14,515	11,396
Postretirement Health Care Benefits
Components of net periodic pension cost and postretirement health care benefit cost
Service cost	336	350	232
Interest cost	4,311	4,292	4,595
Recognized actuarial loss	2,999	4,278	2,825
Net periodic benefit cost	$ 7,646	$ 8,920	$ 7,652