Pension Plan and Retirement Benefits - Defined Benefit Plans - Status (Details) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) person	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)
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		$ 278,280	$ 310,803
Service cost		5,239	5,536	$ 6,282
Interest cost		10,652	10,801	10,577
Actuarial gains (losses)		42,130	(19,756)
Benefits paid		(13,734)	(14,178)
Transfer to third-party insurance company			(13,576)
Administrative expenses		(1,089)	(1,350)
Projected benefit obligation at end of year	$ 321,478	321,478	278,280	310,803
Change in Plan Assets:
Fair value of plan assets at beginning of year		222,273	224,094
Actual return on assets		13,230	18,501
Contribution to plan		5,237	8,782
Benefits paid		(13,734)	(14,178)
Transfer of third-party insurance company			(13,576)
Administrative expenses		(1,089)	(1,350)
Fair value of plan assets at end of year	225,917	225,917	222,273	224,094
Funded Status of Plan:
Unfunded status	(95,561)	(95,561)	(56,007)
Postretirement Health Care Benefits
Change in Benefit Obligation:
Projected benefit obligation at beginning of year		108,013	117,424
Service cost		318	336	350
Interest cost		4,353	4,311	4,292
Actuarial gains (losses)		4,245	(10,395)
Benefits paid		(3,095)	(3,663)
Projected benefit obligation at end of year	113,834	113,834	108,013	$ 117,424
Change in Plan Assets:
Contribution to plan		3,095	3,663
Benefits paid		(3,095)	(3,663)
Funded Status of Plan:
Unfunded status	$ (113,834)	$ (113,834)	$ (108,013)