Pension Plan and Retirement Benefits - Defined Benefit Plans - Status (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($) person	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)
Defined Benefit Pension Plans
Change in Benefit Obligation:
Number of employees that had benefits transferred to a third-party insurance company | person	397
Projected benefit obligation at beginning of year		$ 310,803	$ 337,338
Service cost		5,536	6,282	$ 4,080
Interest cost		10,801	10,577	12,050
Actuarial losses (gains)		(19,756)	(27,246)
Benefits paid		(14,178)	(14,682)
Transfer to third-party insurance company		(13,576)
Administrative expenses		(1,350)	(1,466)
Projected benefit obligation at end of year	$ 278,280	278,280	310,803	337,338
Change in Plan Assets:
Fair value of plan assets at beginning of year		224,094	207,963
Actual return on assets		18,501	25,475
Contribution to plan		8,782	6,804
Benefits paid		(14,178)	(14,682)
Transfer of third-party insurance company		(13,576)
Administrative expenses		(1,350)	(1,466)
Fair value of plan assets at end of year	222,273	222,273	224,094	207,963
Funded Status of Plan:
Unfunded status	(56,007)	(56,007)	(86,709)
Postretirement Health Care Benefits
Change in Benefit Obligation:
Projected benefit obligation at beginning of year		117,424	125,117
Service cost		336	350	232
Interest cost		4,311	4,292	4,595
Actuarial losses (gains)		(10,395)	(8,072)
Benefits paid		(3,663)	(4,263)
Projected benefit obligation at end of year	108,013	108,013	117,424	$ 125,117
Change in Plan Assets:
Contribution to plan		3,663	4,263
Benefits paid		(3,663)	(4,263)
Funded Status of Plan:
Unfunded status	$ (108,013)	$ (108,013)	$ (117,424)