Changes in Accumulated Other Comprehensive Income (Loss) by Component (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Accumulated other comprehensive income (loss)
Balance	$ 303,626	$ 328,624	$ 296,275	$ 333,220
Other comprehensive income (loss)	(2,198)	1,610	3,764	1,019
Balance	303,612	329,692	303,612	329,692
Accumulated Other Comprehensive Income (Loss)
Accumulated other comprehensive income (loss)
Balance	(87,959)	(43,156)	(80,638)	(42,565)
Other comprehensive income (loss) before reclassifications	(3,918)	1,031	325	(140)
Amounts reclassified from accumulated other comprehensive income (loss), Tax benefit	(617)	(207)	(1,231)	(414)
Other comprehensive income (loss)	(2,198)	1,610	3,764	1,019
Balance	(90,157)	(41,546)	(90,157)	(41,546)
Accumulated Defined Benefit Plans Adjustment | Defined Benefit Pension Plans
Accumulated other comprehensive income (loss)
Balance	(60,942)	(21,166)	(53,811)	(21,473)
Amounts reclassified from accumulated other comprehensive income (loss), Tax benefit	(494)	(109)	(987)	(217)
Other comprehensive income (loss)	1,380	306	2,758	613
Balance	(59,562)	(20,860)	(59,562)	(20,860)
Accumulated Defined Benefit Plans Adjustment | Postretirement Health Care Benefits
Accumulated other comprehensive income (loss)
Balance	(17,749)	(10,928)	(13,316)	(11,201)
Amounts reclassified from accumulated other comprehensive income (loss), Tax benefit	(123)	(98)	(244)	(197)
Other comprehensive income (loss)	340	273	681	546
Balance	(17,409)	(10,655)	(17,409)	(10,655)
Amortization of Pension and Postretirement Plan items
Accumulated other comprehensive income (loss)
Amounts reclassified from accumulated other comprehensive income (loss), before tax	52	52	103	103
Amortization of Pension and Postretirement Plan items | Defined Benefit Pension Plans
Accumulated other comprehensive income (loss)
Amounts reclassified from accumulated other comprehensive income (loss), before tax	52	52	103	103
Actuarial losses
Accumulated other comprehensive income (loss)
Amounts reclassified from accumulated other comprehensive income (loss), before tax	2,285	734	4,567	1,470
Actuarial losses | Defined Benefit Pension Plans
Accumulated other comprehensive income (loss)
Amounts reclassified from accumulated other comprehensive income (loss), before tax	1,822	363	3,642	727
Actuarial losses | Postretirement Health Care Benefits
Accumulated other comprehensive income (loss)
Amounts reclassified from accumulated other comprehensive income (loss), before tax	463	371	925	743
Accumulated Translation Adjustment
Accumulated other comprehensive income (loss)
Balance	(9,268)	(11,062)	(13,511)	(9,891)
Other comprehensive income (loss) before reclassifications	(3,918)	1,031	325	(140)
Other comprehensive income (loss)	(3,918)	1,031	325	(140)
Balance	$ (13,186)	$ (10,031)	(13,186)	$ (10,031)
Reclassifications out of accumulated other comprehensive income | Accumulated Other Comprehensive Income (Loss)
Accumulated other comprehensive income (loss)
Amounts reclassified from accumulated other comprehensive income (loss), before tax			(13,283)
Reclassifications out of accumulated other comprehensive income | Accumulated Defined Benefit Plans Adjustment | Defined Benefit Pension Plans
Accumulated other comprehensive income (loss)
Amounts reclassified from accumulated other comprehensive income (loss), before tax			(8,509)
Reclassifications out of accumulated other comprehensive income | Accumulated Defined Benefit Plans Adjustment | Postretirement Health Care Benefits
Accumulated other comprehensive income (loss)
Amounts reclassified from accumulated other comprehensive income (loss), before tax			$ (4,774)