Comprehensive Income (Loss) and Changes in Accumulated Other Comprehensive Income (Loss) by Component - Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Accumulated other comprehensive income (loss)
Balance	$ 333,772	$ 311,299	$ 341,989
Other comprehensive income (loss)	30,129	41,829	(26,570)
Balance	333,220	333,772	311,299
Accumulated Other Comprehensive Income (Loss)
Accumulated other comprehensive income (loss)
Balance	(72,694)	(114,523)	(87,953)
Other comprehensive income (loss) before reclassifications	24,042	31,511
Amounts reclassified from accumulated other comprehensive income (loss), Tax benefit	(2,174)	(6,007)
Other comprehensive income (loss)	30,129	41,829	(26,570)
Balance	(42,565)	(72,694)	(114,523)
Accumulated Defined Benefit Plans Adjustment | Defined Benefit Pension Plans
Accumulated other comprehensive income (loss)
Balance	(43,012)	(74,742)
Other comprehensive income (loss) before reclassifications	17,658	24,109
Amounts reclassified from accumulated other comprehensive income (loss), Tax benefit	(1,381)	(4,426)
Other comprehensive income (loss)	21,539	31,730
Balance	(21,473)	(43,012)	(74,742)
Accumulated Defined Benefit Plans Adjustment | Postretirement Health Care Benefits
Accumulated other comprehensive income (loss)
Balance	(21,691)	(29,585)
Other comprehensive income (loss) before reclassifications	8,284	5,197
Amounts reclassified from accumulated other comprehensive income (loss), Tax benefit	(793)	(1,581)
Other comprehensive income (loss)	10,490	7,894
Balance	(11,201)	(21,691)	(29,585)
Pension and postretirement: Less: amortization of prior service cost
Accumulated other comprehensive income (loss)
Amounts reclassified from accumulated other comprehensive income (loss), before tax	374	808
Other comprehensive income (loss)	275	510	(509)
Pension and postretirement: Less: amortization of prior service cost | Defined Benefit Pension Plans
Accumulated other comprehensive income (loss)
Amounts reclassified from accumulated other comprehensive income (loss), before tax	374	808
Pension and postretirement: Less: amortization of gain (loss)
Accumulated other comprehensive income (loss)
Amounts reclassified from accumulated other comprehensive income (loss), before tax	7,887	15,517
Other comprehensive income (loss)	5,812	9,808	(7,370)
Pension and postretirement: Less: amortization of gain (loss) | Defined Benefit Pension Plans
Accumulated other comprehensive income (loss)
Amounts reclassified from accumulated other comprehensive income (loss), before tax	4,888	11,239
Pension and postretirement: Less: amortization of gain (loss) | Postretirement Health Care Benefits
Accumulated other comprehensive income (loss)
Amounts reclassified from accumulated other comprehensive income (loss), before tax	2,999	4,278
Accumulated Translation Adjustment
Accumulated other comprehensive income (loss)
Balance	(7,991)	(10,196)
Other comprehensive income (loss) before reclassifications	(1,900)	2,205
Other comprehensive income (loss)	(1,900)	2,205	(7,001)
Balance	$ (9,891)	$ (7,991)	$ (10,196)