CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME - USD ($) $ in Thousands		12 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Statement of Comprehensive Income [Abstract]
Net income		$ 132,065	$ 131,672	$ 180,960
Other comprehensive income, net of tax:
Unrealized gain (loss) on available for sale securities, net of tax of $(4,401), $1,499, and $(1,135), respectively	[1]	6,846	(2,187)	1,603
Net unrealized gain on derivatives, net of tax of $0, $0 and $(56), respectively		0	0	93
Adjustment to postemployment benefit obligation, net of tax of $(3,487), $2,466 and $3,688 respectively		5,053	(3,574)	(5,496)
Other comprehensive income (loss)		11,899	(5,761)	(3,800)
Comprehensive income		$ 143,964	$ 125,911	$ 177,160

[1] Available for sale securities are included in other noncurrent assets on the Consolidated Balance Sheets.