Other Comprehensive Income (Loss) - Tax effects of components in other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Sep. 30, 2017	Sep. 30, 2016
Components of Other Comprehensive (Loss) Income:
Net (loss) gain arising during the period	$ (3,012)	$ (7,224)	$ (17,877)	$ 7,271
Accretion of unrealized loss on securities reclassified as held to maturity	(2)	(13)	12	0
Reclassification adjustment for (gain) loss included in net income	60	(116)	2,626	(552)
Total other comprehensive (loss) income, before tax	(2,954)	(7,353)	(15,239)	6,719
Unrealized gain (loss) on swap contract, before tax	192	(2,825)	95	0
Employee benefit plans:
Amortization of prior service cost included in net income	(24)	(623)	(114)	(114)
Reclassification adjustment of actuarial net (loss) gain included in net income	(9)	2,526	11,806	9,123
Change in funded status of retirement obligations	(9,024)	897	11,503	(33,287)
Tax effects resulting from the adoption of ASU No. 2018-02	0	0
Total employee benefit plans, before tax	(9,057)	2,800	23,195	(24,278)
Total other comprehensive (loss)	(11,819)	(7,378)	8,051	(17,559)
Components of Other Comprehensive Income (Loss), Tax Effect
Net (loss) gain arising during the period	(119)	1,446	6,379	(2,597)
Accretion of unrealized loss on securities reclassified as held to maturity	(56)	0	(4)	0
Reclassification adjustment for (gain) loss included in net income	(13)	24	(937)	197
Total other comprehensive (loss) income, tax	(188)	1,470	5,438	(2,400)
Unrealized (loss) gain on swap contract	(30)	595	(33)	0
Employee benefit plans, Tax Effect
Amortization of prior service cost included in net income	(19)	132	41	41
Reclassification adjustment of actuarial net (loss) gain included in net income	(94)	(530)	(4,213)	(3,259)
Change in funded status of retirement obligations	3,354	(776)	(4,106)	11,890
Tax effects resulting from the adoption of ASU No. 2018-02	(10,434)	0
Total employee benefit plans, tax	(7,193)	(1,174)	(8,278)	8,672
Total other comprehensive (loss)	(7,411)	891	(2,873)	6,272
Components of Other Comprehensive Income (Loss), After Tax
Net (loss) gain arising during the period	(3,131)	(5,778)	(11,498)	4,674
Accretion of unrealized gain on securities reclassified as held to maturity	(58)	(13)	8	0
Reclassification adjustment for (gain) loss included in net income	47	(92)	1,689	(355)
Total other comprehensive (loss) income, net of tax	(3,142)	(5,883)	(9,801)	4,319
Unrealized (loss) gain on swap contract	162	(2,230)	62	0
Employee benefit plans, After Tax
Amortization of prior service cost included in net income	(43)	(491)	(73)	(73)
Reclassification adjustment of actuarial net (loss) gain included in net income	(103)	1,996	7,593	5,864
Change in funded status of retirement obligations	(5,670)	121	7,397	(21,397)
Tax effects resulting from the adoption of ASU No. 2018-02	(10,434)	0
Total employee benefit plans, net of tax	(16,250)	1,626	14,917	(15,606)
Other comprehensive loss	$ (19,230)	$ (6,487)	$ 5,178	$ (11,287)