Other Comprehensive Income (Loss) - Tax effects of components in other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Components of Other Comprehensive Income (Loss), Before Tax
Net losses arising during the period	$ (2,892)	$ (23,624)
Accretion of unrealized loss on securities reclassified as held-to-maturity	(2)	0
Reclassification adjustment for (losses) gains included in net income	(60)	411
Total other comprehensive (loss) income, before tax	(2,954)	(23,213)
Unrealized gain (loss) on swap contract, before tax	192	0
Employee benefit plans, Before tax
Amortization of prior service cost included in net income	(24)	(28)
Reclassification adjustment for (losses) gains included in net income	(9)	7
Change in funded status of retirement obligations	(9,024)	20
Tax effects resulting from the adoption of ASU No. 2018-02	0	0
Total employee benefit plans, before tax	(9,057)	(1)
Total other comprehensive loss	(11,819)	(23,214)
Components of Other Comprehensive Income (Loss), Tax Effect
Net losses arising during the period	(147)	8,417
Accretion of unrealized loss on securities reclassified as held-to-maturity	(56)	0
Reclassification adjustment for (losses) gains included in net income	13	(147)
Total other comprehensive (loss) income, tax	(190)	8,270
Unrealized gain (loss) on swap contract, Tax	(28)	0
Employee benefit plans, Tax Effect
Amortization of prior service cost included in net income	(19)	10
Reclassification adjustment of actuarial net (loss) gain included in net income	(94)	(2)
Change in funded status of retirement obligations	3,354	132
Tax effects resulting from the adoption of ASU No. 2018-02	(10,434)	0
Total employee benefit plans, tax	(7,193)	140
Total other comprehensive loss	(7,411)	8,410
Components of Other Comprehensive Income (Loss), After Tax
Net losses arising during the period	(3,039)	(15,207)
Accretion of unrealized loss on securities reclassified as held-to-maturity	(58)	0
Reclassification adjustment for (losses) gains included in net income	(47)	264
Total other comprehensive (loss) income, net of tax	(3,144)	(14,943)
Unrealized gain (loss) on swap contract, Net of tax	164	0
Employee benefit plans, After Tax
Amortization of prior service cost included in net income	(43)	(18)
Reclassification adjustment of actuarial net (loss) gain included in net income	(103)	5
Change in funded status of retirement obligations	5,670	(152)
Tax effects resulting from the adoption of ASU No. 2018-02	(10,434)	0
Total employee benefit plans, net of tax	(16,250)	139
Total other comprehensive loss	$ (19,230)	$ (14,804)