Consolidated Statements of Comprehensive Income (Loss) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 3,678	$ 54,717	$ 22,736	$ 31,072
Other comprehensive income (loss), net of tax:
Unrealized gain (loss) on debt securities available for sale	(3,131)	21,067	(5,778)	(11,498)
Accretion of unrealized gain (loss) on debt securities reclassified as held to maturity	(58)	9	(13)	8
Reclassification adjustment for gain (loss) included in net income	47	2,011	(92)	1,689
Total other comprehensive (loss) income, net of tax	(3,142)	23,087	(5,883)	(9,801)
Derivatives, net of tax:
Unrealized (loss) gain on swap contracts accounted for as cash flow hedges		(6,468)
Unrealized (loss) gain on swap contracts accounted for as cash flow hedges	162		(2,230)	62
Total derivative, net of tax		(6,468)
Total derivative, net of tax	162		(2,230)	62
Employee benefit plans, net of tax:
Amortization of prior service cost included in net income	(43)	(44)	(491)	(73)
Reclassification adjustment of actuarial net (loss) gain included in net income	(103)	(2,930)	1,996	7,593
Change in funded status of retirement obligations	(5,670)	(9,935)	121	7,397
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)	(12,909)	1,626	14,917
Total other comprehensive income (loss)	(19,230)	3,710	(6,487)	5,178
Total comprehensive income (loss), net of tax	$ (15,552)	$ 58,427	$ 16,249	$ 36,250