CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 233,436	$ 132,065	$ 131,672
Other comprehensive income, net of tax:
Unrealized (loss) gain on available for sale securities, net of tax of 6,973, $(4,401) and $1,499, respectively	(19,245)	6,846	(2,187)
Reclassifications of losses to net income on available for sale securities, net of tax of $(858), $0 and $0, respectively	11,647	0	0
Adjustment to postemployment benefit obligation, net of tax of $(573), $(3,487) and $2,466, respectively	1,520	5,053	(3,574)
Other comprehensive (loss) income	(6,078)	11,899	(5,761)
Comprehensive income	$ 227,358	$ 143,964	$ 125,911