Consolidated Statements of Comprehensive Income (Loss) (Unaudited) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 11,784	$ 10,294	$ 15,462	$ 20,286
Other comprehensive (loss) income, net of tax:
Unrealized loss on securities available-for-sale	(9,985)	394	(13,130)	(14,815)
Accretion of unrealized gain on securities reclassified as held-to-maturity	19	0	17	0
Reclassification adjustment for gains included in net income	88	0	28	267
Total other comprehensive (loss) income, net of tax	(9,878)	394	(13,085)	(14,548)
Derivatives, net of tax
Unrealized gain on swap contracts	342	6	505	5
Total derivatives, net of tax	342	6	505	5
Employee benefit plans, net of tax:
Amortization of prior service cost included in net income	(43)	(18)	(85)	(37)
Reclassification adjustment of actuarial net loss included in net income	(102)	5	(205)	11
Change in funded status of retirement obligations	(268)	(135)	(5,876)	17
Tax effects resulting from the adoption of ASU No. 2018-02	0	0	(10,434)	0
Total employee benefit plans, net of tax	(413)	(148)	(16,600)	(9)
Total other comprehensive (loss) income	(9,949)	252	(29,180)	(14,552)
Total comprehensive income (loss), net of tax	$ 1,835	$ 10,546	$ (13,718)	$ 5,734