INCOME TAXES - Deferred tax (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Sep. 30, 2017
Deferred tax liabilities:
Property, plant and equipment	$ 904,734	$ 1,386,512
Available-for-sale securities	10,464	24,940
Other	12,787	21,609
Total deferred tax liabilities	927,985	1,433,061
Deferred tax assets:
Pension reserves	3,477	7,614
Self-insurance reserves	13,100	19,461
Net operating loss, foreign tax credit, and other federal tax credit carryforwards	55,889	62,478
Financial accruals	45,708	62,971
Other	4,888	6,003
Total deferred tax assets	123,062	158,527
Valuation allowance	(48,213)	(58,155)
Net deferred tax assets	74,849	100,372
Net deferred tax liabilities	$ 853,136	$ 1,332,689