CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 169,505	$ 233,436	$ 132,065
Other comprehensive (loss) income, net of tax:
Unrealized (loss) gain on investments in equity securities, net of tax of $0, $6,973 and $(4,401), respectively	0	(19,245)	6,846
Reclassifications of losses to net income on investments in equity securities, net of tax of $0, $(858) and $0, respectively	0	11,647	0
Adjustment to postemployment benefit obligation, net of tax of $6,106, $(573) and $(3,487), respectively	(15,731)	1,520	5,053
Other comprehensive (loss) income	(15,731)	(6,078)	11,899
Comprehensive income	$ 153,774	$ 227,358	$ 143,964