Other Comprehensive Income (Loss) - Tax effects of components in other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Components of Other Comprehensive Income (Loss), Before Tax
Net losses arising during the period	$ (11,594)	$ 651	$ (14,584)	$ (23,012)
Accretion of unrealized loss on securities reclassified as held-to-maturity	(17)	0	(73)	0
Reclassification adjustment for gains included in net income	116	0	55	411
Total other comprehensive (loss) income, before tax	(11,495)	651	(14,602)	(22,601)
Unrealized gain (loss) on swap contract, before tax	382	9	669	9
Employee benefit plans, Before tax
Amortization of prior service cost included in net income	(24)	(28)	(47)	(57)
Reclassification adjustment of actuarial net (loss) gain included in net income	(8)	7	(18)	15
Change in funded status of retirement obligations	33	21	(10,664)	42
Tax effects resulting from the adoption of ASU No. 2018-02			0	0
Total employee benefit plans, before tax	1	0	(10,729)	0
Total other comprehensive (loss) income	(11,112)	660	(24,662)	(22,592)
Components of Other Comprehensive Income (Loss), Tax Effect
Net losses arising during the period	1,609	(257)	1,454	8,197
Accretion of unrealized loss on securities reclassified as held-to-maturity	36	0	90	0
Reclassification adjustment for gains included in net income	(28)	0	(27)	(144)
Total other comprehensive (loss) income, tax	1,617	(257)	1,517	8,053
Unrealized gain (loss) on swap contract, Tax	(40)	(3)	(164)	(4)
Employee benefit plans, Tax Effect
Amortization of prior service cost included in net income	(19)	10	(38)	20
Reclassification adjustment of actuarial net (loss) gain included in net income	(94)	(2)	(187)	(4)
Change in funded status of retirement obligations	(301)	(156)	4,788	(25)
Tax effects resulting from the adoption of ASU No. 2018-02			(10,434)	0
Total employee benefit plans, tax	(414)	(148)	(5,871)	(9)
Total other comprehensive (loss) income	1,163	(408)	(4,518)	8,040
Components of Other Comprehensive Income (Loss), After Tax
Net losses arising during the period	(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)
Unrealized gain (loss) on swap contract, Net of tax	342	6	505	5
Employee benefit plans, After Tax
Amortization of prior service cost included in net income	(43)	(18)	(85)	(37)
Reclassification adjustment of actuarial net (loss) gain 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)