Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Sep. 30, 2017	Sep. 30, 2016
Statement of Comprehensive Income [Abstract]
Net income	$ 3,678	$ 22,736	$ 31,072	$ 32,953
Other comprehensive (loss) income, net of tax:
Unrealized (losses) gains on securities available for sale	(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
Derivatives, net of tax:
Unrealized (loss) on swap contracts	162	(2,230)	62	0
Employee benefit plans, net of tax:
Amortization of prior service cost included in net income	(43)	(491)	(73)	(73)
Reclassification adjustment of actuarial net loss 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	0	0
Total employee benefit plans, net of tax	(16,250)	1,626	14,917	(15,606)
Total other comprehensive (loss) income	(19,230)	(6,487)	5,178	(11,287)
Total comprehensive income (loss), net of tax	$ (15,552)	$ 16,249	$ 36,250	$ 21,666