Other Comprehensive Income (Loss) - Changes in components of 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
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of year			$ 475,914	$ 439,664
Current period changes in other comprehensive (loss) income	$ (9,949)	$ 252	(17,472)	(14,552)
Tax effects resulting from the adoption of ASU No. 2018-02			(11,708)	0
Balance at end of year	473,904	445,398	473,904	445,398
Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of year	(65,411)	(66,162)	(46,180)	(51,358)
Balance at end of year	(75,360)	(65,910)	(75,360)	(65,910)
Total other comprehensive (loss) income	(75,360)		(75,360)
Unrealized Gains on Securities Available-for- Sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of year	(7,345)	(9,279)	(4,075)	5,664
Current period changes in other comprehensive (loss) income	(9,878)	394	(11,771)	(14,548)
Tax effects resulting from the adoption of ASU No. 2018-02			(1,314)	0
Balance at end of year	(17,223)	(8,885)	(17,223)	(8,885)
Total other comprehensive (loss) income	(17,160)	(8,884)	(17,160)	(8,884)
Swaps
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of year	224	0	0	0
Current period changes in other comprehensive (loss) income	342	6	465	5
Tax effects resulting from the adoption of ASU No. 2018-02			40	0
Balance at end of year	566	6	566	6
Total other comprehensive (loss) income	505	5	505	5
Employee Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of year	(58,290)	(56,883)	(42,105)	(57,022)
Current period changes in other comprehensive (loss) income	(413)	(148)	(6,166)	(9)
Tax effects resulting from the adoption of ASU No. 2018-02			(10,434)	0
Balance at end of year	(58,703)	(57,031)	(58,703)	(57,031)
Total other comprehensive (loss) income	$ (58,705)	$ (57,031)	$ (58,705)	$ (57,031)