Changes in Accumulated Other Comprehensive Income (Loss) by Component - Accumulated 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
Accumulated other comprehensive income (loss)
Balance			$ 333,772
Other comprehensive income (loss)	$ 3,627	$ 3,414	5,449	$ 2,968
Balance	310,205		310,205
AOCI Attributable to Parent
Accumulated other comprehensive income (loss)
Balance	(70,871)	(114,969)	(72,694)	(114,523)
Other comprehensive income (loss) before reclassifications	2,316	835	2,833	(2,191)
Amounts reclassified from accumulated other comprehensive income (loss), Tax benefit	(755)	(1,502)	(1,515)	(3,004)
Other comprehensive income (loss)	3,626	3,414	5,449	2,968
Balance	(67,245)	(111,555)	(67,245)	(111,555)
Accumulated Defined Benefit Plans Adjustment | Defined Benefit Pension Plans
Accumulated other comprehensive income (loss)
Balance	(42,180)	(72,838)	(43,012)	(74,742)
Amounts reclassified from accumulated other comprehensive income (loss), Tax benefit	(477)	(1,109)	(961)	(2,217)
Other comprehensive income (loss)	838	1,903	1,670	3,807
Balance	(41,342)	(70,935)	(41,342)	(70,935)
Accumulated Defined Benefit Plans Adjustment | Postretirement Health Care Benefits
Accumulated other comprehensive income (loss)
Balance	(21,217)	(28,909)	(21,691)	(29,585)
Amounts reclassified from accumulated other comprehensive income (loss), Tax benefit	(278)	(393)	(554)	(787)
Other comprehensive income (loss)	472	676	946	1,352
Balance	(20,745)	(28,233)	(20,745)	(28,233)
Amortization of Pension and Postretirement Plan items
Accumulated other comprehensive income (loss)
Amounts reclassified from accumulated other comprehensive income (loss), before tax	94	202	187	404
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	94	202	187	404
Actuarial losses
Accumulated other comprehensive income (loss)
Amounts reclassified from accumulated other comprehensive income (loss), before tax	1,971	3,879	3,944	7,759
Actuarial losses | Defined Benefit Pension Plans
Accumulated other comprehensive income (loss)
Amounts reclassified from accumulated other comprehensive income (loss), before tax	1,221	2,810	2,444	5,620
Actuarial losses | Postretirement Health Care Benefits
Accumulated other comprehensive income (loss)
Amounts reclassified from accumulated other comprehensive income (loss), before tax	750	1,069	1,500	2,139
Accumulated Translation Adjustment
Accumulated other comprehensive income (loss)
Balance	(7,474)	(13,222)	(7,991)	(10,196)
Other comprehensive income (loss) before reclassifications	2,316	835	2,833	(2,191)
Other comprehensive income (loss)	2,316	835	2,833	(2,191)
Balance	$ (5,158)	$ (12,387)	$ (5,158)	$ (12,387)