Other Comprehensive Income (Loss) - Changes in components of other comprehensive income (loss) (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
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	(7,522)	(14,804)
Reclassification of tax effects resulting from the adoption of ASU No. 2018-02	(11,708)
Balance at end of year	472,070	434,853
Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of year	(46,180)	(51,358)
Balance at end of year	(65,410)	(66,162)
Unrealized Gains on Securities Available for Sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of year	(4,135)	5,664
Current period changes in other comprehensive (loss) income	(1,830)	(14,943)
Reclassification of tax effects resulting from the adoption of ASU No. 2018-02	(1,314)
Balance at end of year	(7,279)	(9,279)
Employee Benefit Plans
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of year	(42,105)	(57,022)
Current period changes in other comprehensive (loss) income	(5,816)	139
Reclassification of tax effects resulting from the adoption of ASU No. 2018-02	(10,434)
Balance at end of year	(58,355)	(56,883)
Swaps
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of year	60	0
Current period changes in other comprehensive (loss) income	124	0
Reclassification of tax effects resulting from the adoption of ASU No. 2018-02	40
Balance at end of year	$ 224	$ 0